Ukrainian emergency crews rushed to evacuate the most vulnerable on the western side of the river, while conservationists warned that a huge and long-lasting environmental disaster was unfolding.

It was more difficult to assess what was happening on the eastern bank of the river south of the dam, which is under Russian control. But more than 40,000 people could be in the path of the flooding on both Ukrainian- and Russian-controlled territory, the deputy prosecutor general of Ukraine, Viktoriya Lytvynova, said.

It was not immediately clear who was responsible for the destruction of the Kakhovka dam and electric plant, which lies along the Dnipro River and is held by Russian forces. President Volodymyr Zelensky blamed “Russian terrorists,” while the Kremlin’s spokesman, Dmitri S. Peskov, said Ukrainian forces carried out a “sabotage” attack.

The disaster came one day after American and Russian officials said a planned Ukrainian counteroffensive might have begun east of the Dnipro in the Donetsk region. Though the dam is far from that fighting, its destruction could divert both sides’ resources from the counteroffensive.

The dam creates a reservoir that supplies water for drinking and agriculture. It also provides water to cool reactors and spent fuel at the Zaporizhzhia Nuclear Power Plant, though the United Nations’ nuclear watchdog, the International Atomic Energy Agency, said there was “no immediate nuclear safety risk.” The group said, however, that it was closely monitoring the situation.

Security of the dam, the second largest on the Dnipro, had been a continuing concern during the war, with both sides accusing the other of plotting to destroy it.

Here are other developments:

• This is what we know so far about the Ukraine dam disaster.

• Videos and images on social media showed flooding already underway in communities downstream from the Kakhovka dam, and streets filling with rising water. In Nova Kakhovka, the city under Russian control that lies immediately downstream of the dam, the Palace of Culture and administrative center were swamped.

• An energy official said floodwaters across southern Ukraine caused by the destruction of the Kakhovka dam are expected to continue to rise through the night and peak on Wednesday morning.

• In Mykolaiv, an emergency train collected people fleeing the rising waters in Kherson, about 40 miles to the east. Humanitarian groups were just starting to arrive to provide support for those forced from their homes by flooding.

• Russian officials said the destruction of the dam potentially poses problems for a canal supplying water to Crimea that has for years been a point of geopolitical tension between Kyiv and Moscow.

Marc Santora, Maria Varenikova and Anna Lukinova contributed reporting.

But even for Kherson, she soon realized Tuesday morning, things were far from normal.

Water was filling the streets of her low-lying neighborhood — and rising quickly. A dam had been destroyed, and soon the power went out, the gas stopped working and the water supply to her apartment stopped flowing.

So Ms. Alfiorova did something she had long resisted despite all the hardships of the past year and a half: She fled. She boarded an evacuation train from Kherson to Mykolaiv, about 40 miles to the west, stepping out onto Platform 1, homeless for the first time in her life.

“I had no choice,” she said.

Many of her neighbors and friends, however, decided to take their chances and stay, and on the train meant to take people to safety, there were only 43 passengers, among them several children. Most of the 10 cars were empty.

Ms. Alfiorova said many people she knew had decided to move to higher ground to stay with friends and family or to ride out the floods in apartments on high floors.

“I have a neighbor on the third floor and she has three dogs,” she said. “She is not going to leave her home.”

She herself lives on the fourth floor of the nine-story building, and for her, the flooding was one hardship too many, though it is the just latest sorrow for a city that was home to 290,000 people before Russia invaded last year.

Ms. Alfiorova, a sociologist, recalled the grim months of Russian occupation, when she had little money or food. Soldiers menaced civilians, seeking out those with pro-Ukrainian sympathies, looting homes and businesses, and failing to deliver even the most basic services to people.

The threat did not fully lift after Ukrainian forces recaptured Kherson in November and the Russians took to shelling the city from afar. Ms. Alfiorova became so used to it that she learned to measure the danger by the sounds in the air.

“If I hear a whistle, it can be quite far,” she said. “If it is whistling I know it is not for my soul. But when it is a rumbling sound, you realize that it will land quite close.”

In March, she said, a shell exploded so close that she thought for a moment it could be the end. But she survived.

On Tuesday, when explosions boomed once again around 4 a.m. she figured it was just the usual Kherson wake-up call. It was not. “The neighbors were screaming,” she said.

As the streets disappeared under a coursing tide of water, police cars began patrolling with loudspeakers to warn of the growing danger. Evacuate, residents were urged.

“I checked the Telegram channels, talked to neighbors and friends and decided to go,” Ms. Alfiorova said. She and her son, Oleh, 23, raced to gather important documents, a few cherished possessions and her two cats, Biusia and Miusia, whom she placed in cardboard pet carriers.

But when they tried to make it out of their neighborhood, the shelling resumed, forcing them to take cover in a basement. Only when it subsided could they make their way to the train station.

“As we were leaving, we realized we forgot all of our money,” Ms. Alfiorova said. But there were teams of volunteers from a host of aid agencies at the train station to help her.

She has checked back with friends who stayed behind and believes she made the only decision she could, however hard. “The level of the water is so high now, people can swim,” she said.

Similar scenes were described in Antonivka, about 40 miles downstream from the destroyed dam.

One resident of the town, Hanna Zarudnia, 69, said she had spent the night in a basement bunker because of intense shelling. “About 10 houses were damaged,” she said. “Roofs were destroyed.”

Then a new horror took shape.

“Antonivka was surrounded by water from all sides, we were on an island,” she said. “I have pictures, videos: roads, a stadium, a school were flooded, everything came under water.”

Ukraine and Russia have each accused the other of blowing up the dam, a critical structure whose breach has put thousands of people downstream at risk.

Ms. Zarudnia scoffed at the notion that Ukraine blew up its own dam, and recalled that similar claims were made about attacks in Kherson, where she once lived under occupation. “I was a witness to that,” she said.

She has no doubt who was bombing her home week after week back then, she said, and none about who blew up the dam now.

John Kirby, a spokesman for the National Security Council, said the United States has been monitoring the effects of the destruction of the Kakhovka dam but that he could not confirm news reports that Russia was responsible. “We are working with the Ukrainians to gather more information,” Kirby said. “We know there are casualties, including likely many deaths, though these are early reports and we cannot quantify them.”

Asked if the U.S. would consider the destruction to be a war crime, Kirby said it was too early to determine. But he stressed that Russia was illegally occupying the dam at the time of the explosion. “It’s very clear that the deliberate destruction of civilian infrastructure is not allowed by the laws of war," he said.

A large pond next to the Kakhovka reservoir contains enough water to cool the reactors at the Zaporizhzhia Nuclear Power Plant for “some months,” lessing the immediate risk posed to the plant when the reservoir's dam was destroyed on Tuesday, the International Atomic Energy Agency said in a statement. “It is therefore vital that this cooling pond remains intact," the statement said. "Nothing must be done to potentially undermine its integrity."

A team of U.N. inspectors based at the nuclear plant will continue to monitor the situation and the agency’s director, Rafael Mariano Grossi, plans to visit the nuclear power plant next week, the statement said.

The HALO Trust, a British American charity that clears land mines, said that it has been conducting operations in areas now affected by the flooding. The group has been clearing mines that were planted by Russian troops to prevent Ukrainians from crossing the Inhulets River, a front line last year.

“These mines are now posing a fatal risk to civilians who are returning to their homes or use the fertile banks to graze their animals, cultivate crops and fish,” said Jasmine Dann, the charity’s location manager for the Mykolaiv region.

Ms. Dann said that the group had already seen a 21-inch rise in the river level on Tuesday since the flooding began. With many mines sitting close to the surface, the force of the current could wash them away or cause explosions while they are in the water, she added.

The teams regularly cross the river to clear minefields, she added, and more than 460 mines had been discovered near riverbanks. If river levels rose significantly, their teams would be cut off from access.

Daria Shulzik, 38, who lives in a town downstream from the dam, said she worried that the waters would dislodge land mines and spread them about.

The Russian military, she said, had created a disaster in her region. “I don’t know why they started this war, and why they carry on,” she said, adding: “Agriculture will suffer, and the Black Sea will suffer because all this is flowing into the sea,” she said.

For years, it served as Crimea’s main water resource, but shortly after the annexation, Ukraine blocked the flow of water. Russia restored it after invading last year and occupying the territory around the canal.

The Kremlin’s spokesman, Dmitri S. Peskov, said in a briefing on Tuesday that water levels in the reservoir were falling as a result of the dam’s destruction, reducing supply to the canal. Only a small portion of the canal’s water supply is used for drinking water. Most of it is used for agricultural purposes in Crimea.

Sergei Aksyonov, the Kremlin-installed leader of Crimea, said on Tuesday that there was a risk of the water in the canal becoming shallow.

Writing on the Telegram messaging app, Mr. Aksyonov said that 40 million cubic meters of water remained in the canal and that work was being done to minimize losses. He said reservoirs in Crimea were 80 percent full, adding that there was sufficient drinking water for the peninsula’s residents.

“In the coming days, the situation will be clear, as well as the possible risks,” Mr. Aksyonov wrote.

Tuesday’s image shows a breach at the dam in three locations. About 200 yards of the dam’s central area have been destroyed, and a structure at the hydroelectric power plant sitting atop the dam is split in half. A drone video taken earlier in the day showed part of the southern end of the dam still intact. However, only a few hours later, that area was underwater, according to the satellite image.

Floodwaters have reached at least two miles downstream of the dam, where a marina and sports facilities in the town of Nova Kakhovka have disappeared completely under the water, according to the satellite image.

“We expect the peak to be between tonight and tomorrow morning,” he said. “After that, in two days, it will start to decline, and we understand that within 10 days this water will be gone and we will see the consequences of this disaster.”

It is impossible to stop the flow of water and the rate is only likely to increase, he said, adding that the dam’s electric plant can’t be repaired. “The lower part of it has already been washed away,” he said.

The loss of the dam will not severely affect the country’s energy grid, he said, because the hydroelectric plant, which has been under Russian occupation since March of last year, has not been operating in the power grid since October.

But it will cause a severe shortage of drinking water in the Dnipro, Zaporizhzhia and Kherson regions, he said.

Though the destruction of the Kakhovka dam and its hydroelectric plant has immediate humanitarian and strategic implications, it will have little bearing on Ukraine’s energy grid. Ukraine derives most of its electricity from the three nuclear plants still under its control. Before it was occupied by Russian troops, the Kakhovka plant accounted for around 2 percent of Ukraine’s electricity, said Alex Riabchyn, Ukraine’s former deputy energy minister.

Because of the war’s toll on Ukraine’s economy, electricity usage is far lower than it once was — so much so that Ukraine exported small amounts of electricity from its grid to nearby parts of Europe last summer.

The Kinburn Spit, which lies at the mouth of the Dnipro River and is controlled by Russian forces, could become an island as a result of flooding caused by the destruction of the Kakhovka dam, the spokeswoman for Ukraine’s southern forces, Natalia Humeniuk, said on Tuesday. This would “complicate the enemy’s logistics,” she said. The spit is a target for Ukrainian forces partly because of its proximity to the Crimea region, which was illegally annexed by Russia in 2014.

If Ukraine gained control of the Spit, it would gain access to the land bridge east of the Dnipro River that is controlled by Russian forces. It would also make it easier to mount any future attack on Crimea. The two sides have traded fire across the water north of the Kinburn Spit since November, when Ukraine retook the city of Kherson.

“It will have a series of acute and also long-term environment effects,” said Doug Weir, the research and policy director at the Conflict and Environment Observatory, a nonprofit organization based in Britain. “It’s going to have an enormous legacy.”

Around the dam itself, species like fish could be spawning in the shallows, Mr. Weir said, adding, “The sudden drop in water level is going to expose those areas and impact habitats.” The large volume of water rushing down the river could also damage fragile sand banks, reed beds and other ecosystems along the river.

The Dnipro River and its surrounding land were already home to an array of nature reserves. There is Oleshky Sands National Nature Park, Europe’s second-largest expanse of sand, and the Black Sea Biosphere, a UNESCO-designated reserve of wetlands and marshes. And further downstream, there is the Nizhnyodniprovskyi National Nature Park, also known as the Lower Sula National Park, a protected area home to a vast array of flora and fauna, among them endangered plants and rare birds.

Concerns were also heightened over industrial pollution, with the Ukrainian government saying that 150 tons of machine oil had leaked into the Dnipro River, and that 300 more tons were at risk of seeping into the waterway. That could pose a toxic catastrophe to the river’s fish, which had already been decimated by the effects of the war.

Rising water levels could also lead to leaks from fuel stations, septic tanks and industrial sites around the shipyards of Kherson. “All these things can become point sources of pollution if they are inundated, which looks likely to be the case,” Mr. Weir said.

Kherson

The authorities have ordered residents to leave the city, which lies about 37 miles southwest of the dam. In November, Ukrainian forces recaptured Kherson on the west bank of the river. It was a significant victory for Ukraine. Since then, however, the city and the surrounding area on the river’s western side have been repeatedly shelled by Russian forces. Many civilians have been killed, and the resumption of normal life has been all but impossible.

Dnipro River basin

Downstream of the dam, toward the Black Sea basin, lie smaller rivers and islands, as well as towns and fishing villages on both sides of the river. These include the town of Oleshky and the Oleshky Sands National Nature Park on the eastern side. Residents have been ordered to evacuate the area, which includes large tracts of rich, irrigated agricultural land. The Ukrainian authorities have said that Russian forces mined the river’s east bank to hinder their forces from crossing.

Kakhovka Reservoir

The dam’s destruction is draining water into the Black Sea from the Kakhovka Reservoir, a body of water that is 10 miles wide in places. There are a string of villages on the reservoir’s banks. Nikopol, on the western bank, is the largest city. It has been repeatedly shelled by Russian forces stationed on the river’s eastern bank. Ukraine holds the western side of the river, while Russian forces control part of the eastern side.

Zaporizhzhia Nuclear Power Plant

The plant, the largest in Europe, is the most critical infrastructure on the reservoir. It sits on the east bank of the river, near the city of Enerhodar and draws water from the reservoir to cool its six reactors. Both Russia and Ukraine have for months been building up their forces along a front line east of the reservoir to prepare for a Ukrainian counteroffensive. The front line in the Zaporizhzhia region begins around 30 miles northeast of Enerhodar.

More than 40 miles downstream of the dam, another video captures the scale of the threat to islands in the Dnipro River. Homes on the eastern tip of Potemkin Island appear to be almost entirely underwater.

That island lies just southwest of the city of Kherson, where flooding had also begun to creep into the streets on Tuesday morning. Video shows rising water levels at the city’s Slavy Park, with smoke rising in the distance.

Five of the six reactors at the plant, which is the largest civilian nuclear facility in Europe, have been shutdown for months, meaning they require a relatively small amount of water. The sixth reactor is cooled with water from the large on-site pond fed from the gushing reservoir, but the pond itself is full and secure, said Ulrich Kühn, a nuclear expert at the University of Hamburg and the Carnegie Endowment for International Peace.

“The reservoir will lose a lot of water due to the damaged dam, but right now this is not a big problem for Z.N.P.P.,” Mr. Kühn said, using the plant’s acronym. “The situation is under control and not critical.”

For months, the I.A.E.A.’s director general, Rafael Mariano Grossi, has sounded the alarm about the potential for nuclear catastrophe at the plant. On Tuesday the agency said on Twitter that, while it was “closely monitoring” the situation surrounding the dam, there was “no immediate nuclear safety risk.” Mr. Grossi also said that he plans to visit the plant, where his agency has stationed inspectors, next week.

Ukraine’s state nuclear company, Energoatom, also said there was no immediate danger, and suggested that the pond had enough water for the foreseeable future.

“The station’s cooling pond is full,” it said in a post on the Telegram social messaging app. “At 8 a.m. the water level is 16.6 meters, which is sufficient for the station’s needs.”

Russian forces have occupied the nuclear plant since the early weeks of their full-scale invasion of Ukraine, which began more than 15 months ago. The possibility of a breach in the Kakhovka dam has been an ever-present risk to safety at the facility because of the cooling issue, though shelling has been a more immediate threat.

Water from the Kakhovka reservoir feeds the plant’s cooling pond, said Ivan Plachkov, a former energy minister of Ukraine. The facility currently requires 130,000 to 260,000 gallons of water to circulate per hour to maintain safety and critical functions, he said.

Reserve sources of water exist on site but if water is not circulated, the water in the pool could boil and evaporate. That could emerge at some stage as a longer-term risk, but it is not an immediate problem because the pond would most likely only evaporate slowly over time as a result of the sun’s heat.

The cooling pond’s level “is sufficient for the plant’s needs, but over time the water from the pond may evaporate and, in case it couldn’t be filled, it wouldn’t be possible to operate the plant,” said Maria Kurando, a visiting doctoral fellow at the University of Hamburg’s Institute for Peace Research and Security Policy.

The American Nuclear Society, an international professional organization, provided additional details on how safety at the plant could be managed in the light of the draining reservoir.

“The nuclear plant has mobile pumping units that can be used to access water from alternative sources,” it said in a statement. “The plant also has special floating water intakes which allow the facility to draw water when the reservoir is at low levels.”

Moscow has stationed its forces and military equipment at the nuclear facility. Repeated shelling last summer, for which Russia and Ukraine blamed each other, damaged parts of the plant, including an area where spent fuel is stored.

More recently, the plant has been forced to rely on diesel generators at least half a dozen times as workers have switched off external power in response to shelling at power plants elsewhere.

At the same time, the Russian authorities in control of the plant have put pressure on the Ukrainian employees that still work there under occupation to sign contracts with Russia’s state nuclear firm, Rosatom. Workers say that they have been detained, beaten and tortured.

Moscow’s control of the plant has given it leverage over Ukraine’s energy system and it no longer supplies to Ukraine’s national grid.

“I heard a boom, and my windows shook,” Tatyana Yeroshenko, 32, a teacher of Ukrainian, said in a telephone interview.

“What do they want from us?” she asked. “They want us to be with them, to unite us with Russia. But we aren’t happy with that idea. We will never be part of Russia.”

The head of the Kherson region military administration, Oleksandr Prokudin, said that in total about 16,000 people lived in towns and villages at risk from flooding in Ukrainian-controlled areas on the west bank of the Dnipro River. It was not immediately clear how many people were at risk in Russian-held areas on the east bank.

The shelling of communities already flooded by the burst dam shed light on the sweeping scale of the disaster of Russia’s war in Ukraine, in which tragedies pile on top of tragedies.

The Ukrainian authorities were evacuating people by train and bus on Tuesday. As the floodwaters rose, swamping houses, spilling over farm fields and blocking roads, they also flooded a zoo in the town of Nova Kakhovka, according to the city’s mayor, Volodymyr Kovalenko.

In telephone interviews arranged by a group distributing humanitarian aid in Antonivka, residents described how they had watched as rising waters crept from house to house. They kept their distance from the river bank, where Russian snipers on the opposite side have in the past fired at residents, they said.

In this area of southeastern Ukraine, where the Dnipro River forms the front line, the floodwaters were flowing into towns where tens of thousands of people had already evacuated because of the war. In Antonivka, for example, about 4,000 residents remained before the flooding Tuesday, out of a prewar population of about 13,000, according to Ms. Yeroshenko, who is also a volunteer at the aid group, which had estimated the wartime population.

Ms. Yeroshenko woke up around 5 a.m. to explosions from artillery, a common occurrence in her town, she said. She said she checked her phone for news and saw reports that the Russians had also caused a major flood. “The water is rising,” she said later that morning. It had already inundated the town’s soccer stadium, she said. As well as the threat to human life, she said the disaster could result in “an ecological catastrophe.”

Daria Shulzik, 38, an office manager, said she awoke to an unusual noise from the river that sounded like pouring rain. The water was muddy and flowing past, she said, and there were “a lot of dirt, branches, parts of buildings, fences, cattails from swamps — everything.” The flooding was blocking roads, she said.

Ms. Shulzik also worried that the water would dislodge land mines planted in abundance on both banks of the rivers by both Russia and Ukraine’s armies and spread them about in the floodwater.

The Russian military, she said, had created a disaster in her region. “I don’t know why they started this war, and why they carry on,” she said, adding: “agriculture will suffer, and the Black Sea will suffer because all this is flowing into the sea,” she said. “Even the fish will suffer now.”

Yevhen Chupyna, a Red Cross rescue worker, said that the scale of the disaster had yet to come into focus for many living in areas that might be flooded.

“The situation is difficult emotionally and psychologically,” he said as he helped unpack boxes of humanitarian aid. “People don’t really know what happened. They have not realized this is a catastrophe.”

With communications spotty, he said it was difficult to get accurate information about the state of the flooding. The city of Kherson straddles the Dnipro River, which has become a front line in the war, dividing the warring armies.

The western bank, which is where the majority of Kherson’s residents live and work, is controlled by Ukraine, retaken last fall after eight months of Russian occupation. It mostly sits on elevated land but there are some neighborhoods close to the river bank where flooding has already been reported. The eastern bank, controlled by the Russians, is kind of a bayou, with islands and marshes and many country homes accessible only by boat, even before the dam was breached.

Ukrainian officials, citing reports from emergency workers and volunteers, said some neighborhoods near the river were already flooded. Vasyl, 40, a factory worker who lives in Kherson, said in a brief text message that people were trying to evacuate low lying neighborhoods but that Russians were still shelling the area.

“Russians opened mortar fire as people prepared to evacuate from Ostriv,” he wrote. “They are terrorizing us.”

Alim, who reached out from Kherson via text message, said people in the lower part of the city were in a panic. “Some are moving stuff to the upper floors and roofs of their houses, while others are packing the cars and trying to leave,” he wrote.

Buses were being organized to take people from their home to the train station, but only about 30 people were registered to take the first 10-car train as of 12 p.m. local time. Mr. Chupyna said that they have hundreds of beds in Mykolaiv prepared for people who were forced from their homes.

Over 15 months of war, Ukrainian volunteer organizations have become adept at responding quickly to emergencies. But rising waters from a breached dam was a totally new challenge. Olha Napkhanenko, 40, a volunteer with the Serhiy Prytula Charity Foundation, said that her colleagues in the city of Kherson reported only about 5 percent of the city being severely affected as of noon, but that the situation could get worse.

As she prepared snacks for the children who might arrive, the Ukrainian national anthem echoed through the station hall as workers stacked supplies.

“The worst will be on the eastern bank,” she said, referring to Russian-occupied territory. “Unfortunately, we can’t help them.”

Svitlana Sitnik, 52 a volunteer from a different organization, said her aunt was in one of the towns on the east bank occupied by the Russians, Oleshky, and she was in contact with people there via a private Telegram channel. They painted an increasingly dire situation for the civilians there as Russian soldiers continued to patrol the streets and refused to provide assistance as the waters rose

The Russians announced an evacuation plan, people in the city reported, but details were scarce about how it would work.

For now, Ms. Sitnik said, it was neighbors helping neighbors in Oleshky. “Local volunteers are offering to help people get to Crimea,” she said.

But internet and cellular service was spotty and even if they could use their phones, she said, people are afraid to use them on the streets for fear of attracting the attention of Russian soldiers. “All the people are exhausted,” she said as she shared with a reporter the conversations on the secure Telegram channel on her phone. “They are on the edge. They have no rights.”

Russian forces shelled the southern Ukrainian region of Kherson on Tuesday and wounded two police officers while the area was being evacuated, Ukraine’s minister of internal affairs, Ihor Klymenko, said on national television. Russian forces have shelled the river’s western side regularly since they withdrew from the city of Kherson in November.

The dam’s destruction also increases the risk posed by land mines, Klymenko said, referring to the prospect that flooding could wash mines from their original positions. The Ukrainian authorities have previously accused Russia of mining the eastern bank of the Dnipro River to impede any attempt by Ukrainian forces to cross the river as part of a counteroffensive.

The security of the dam, the second largest of the cascade of dams on the Dnipro River and a vital source of water and power, has been a continuing concern during the war in Ukraine, with both sides accusing the other of plotting to destroy it.

Mr. Peskov said one of the reasons Ukraine would have for attacking the dam would be to deprive Crimea of water. The reservoir above the destroyed dam connects to a canal supplying water to the Crimean peninsula, which Russia annexed illegally in 2014.

Mr. Peskov denied accusations by Ukrainian officials that Russian forces had blown up the dam.

“All responsibility for all the consequences should rest with the Kyiv regime,” he said.

Ukraine’s hydropower company, Ukrhydroenergo, said the damage at the dam was caused by an explosion inside the engine room, which is under Russian control. The power plant, it said, “cannot be restored.”

Mr. Zelensky condemned the destruction of the dam as an act of terrorism and blamed Russian forces, which he vowed to push out of Ukraine. Ukraine has called for an urgent meeting of the U.N. Security Council to discuss the attack, the ministry of foreign affairs said in a statement.

Most experts think the counteroffensive will be east of the river, but the destruction of the dam could divert both sides’ attention and resources.

The Dnipro forms the front line between Ukrainian and Russian forces from Zaporizhzhia all the way to the Black Sea. The dam is the southernmost along the Dnipro and is roughly 35 miles east of the Ukrainian-controlled port city of Kherson.

Since Ukrainian forces retook Kherson in November, Kyiv’s troops have mounted limited river crossings toward the Russian-controlled eastern bank but have failed to gain a significant foothold.

The destruction of the dam, and subsequent flooding, would make any future cross-river incursions downriver from the dam more difficult for Ukrainian forces. Russian troops most likely fear Ukrainian incursions as they try to defend a roughly 600-mile-long front line.

The flooding will also draw resources away from the Ukrainian government as it scrambles to evacuate civilians trapped in the flood zone, potentially affecting the counteroffensive, depending on the scope of destruction.

The same could be said about flooded Russian-occupied territory on the eastern bank, where entrenched Kremlin troops could be displaced by rising water. Minefields and weapons storage areas could also be affected.

Pro-Russia military bloggers immediately blamed Kyiv for damaging the dam, claiming that the facility’s destruction was the result of constant shelling by Ukrainian forces dating back to last year.

Natalia Humeniuk, a spokeswoman for Ukraine’s southern command, said it was clear that the explosion “took place from the inside” and was not a rocket attack.

“They decided that now, in this way, they will be able to stop the counteroffensive of Ukrainian forces,” Ms. Humeniuk told Radio Svoboda Tuesday.

Igor Girkin, a former Russian paramilitary leader who goes by the pseudonym Igor Strelkov, wrote on the Telegram messaging app that the reduction in water levels upriver from the dam would force Russia to send reserves to the area to prevent Ukrainian troops from trying to ford the river there and create a bridgehead.

Dam warfare is nothing new for Russia. In 1941, during World War II, Soviet forces destroyed the Dnipro hydroelectric plant and dam, located north of Kakhovka, to thwart a German advance, killing thousands of civilians in the process. Two years later, the Germans would do the same, this time as they retreated west.

The early stages of what could be Ukraine’s counteroffensive has been marked with feints and distractions, most notably with a recent series of Ukrainian-backed cross-border incursions into Russia’s Belgorod region.

Those skirmishes have been minor compared with the heavier fighting in the south and the east. There, U.S. officials have noticed an uptick in activity and unverified videos posted online have shown large amounts of Western-supplied equipment in several larger Ukrainian assaults.

Russia’s pro-military bloggers blamed Ukraine, without providing evidence, for destroying the dam. Some of them suggested that a drop in water levels upstream, in the regions of Kherson and Zaporizhzhia, could aid Ukrainian forces in crossing where the Dnipro River forms the front line of fighting.

Igor Girkin, a former paramilitary leader who writes under the name Igor Strelkov, said on Telegram that Russian forces would need to transfer additional reserves upriver to defend the territory and prevent Ukrainian troops from crossing and creating a bridgehead in the coming weeks.

In a sign of how widely the dam’s destruction may be felt, the Kremlin-backed head of Crimea issued a warning about the water levels of the North Crimea Canal, which supplies fresh water to the peninsula from the Dnipro river.

In a statement on Telegram, Sergei Aksyonov said that Crimea had enough water reserves in its reservoirs, but that water levels could drop. The situation will become clearer in the coming days, he said.

The U.N. secretary general, Antonio Guterres, decried the destruction of the Kakhovka hydroelectric dam, calling it a “monumental humanitarian, economic and ecological catastrophe” and “yet another example of the horrific price of war on people.”

The dam is near the front line of the war.

Videos of the dam, in the town of Nova Kakhovka, reviewed by The New York Times do not reveal what caused the destruction. But they do show water flowing freely through the dam, indicating severe damage.

The disaster came one day after American and Russian officials said a planned Ukrainian counteroffensive appeared to have begun east of the Dnipro in the Donetsk region. The flooding could divert both sides’ attention and resources from that counteroffensive.

Located near the front line of the war in the southern Kherson region, the dam and nearby infrastructure have been damaged by shelling throughout the war. Last year, Russian forces took control of the dam and a nearby hydroelectric plant. The Ukrainians now say the power plant “cannot be restored.”

Russia and Ukraine traded blame.

On Tuesday, Russia and Ukraine blamed each other for the destruction, without offering evidence.

President Volodymyr Zelensky of Ukraine blamed “Russian terrorists,” while the Kremlin’s spokesman, Dmitry S. Peskov, blamed Ukrainian forces, describing what happened as a “sabotage” attack.

“They decided that now, in this way, they will be able to stop the counteroffensive of Ukrainian forces,” Natalia Humeniuk, a spokeswoman for Ukraine’s southern command, told Radio Svoboda Tuesday.

Sergei K. Shoigu, Russia’s defense minister, accused Ukraine of destroying the dam, saying Kyiv wanted to move forces and equipment defending Kherson to other parts of the front to help with its counteroffensive.

Security of the dam, a vital source of water and power, has been a continuing concern during the war, with both sides accusing the other of plotting to destroy it.

Thousands of people are at risk.

Communities along the waterway are at risk of being flooded and washed away. About 16,000 people are in the “critical zone” on the Ukrainian-controlled western bank of the Dnipro River, said Oleksandr Prokudin, the regional military administrator. Another 25,000 people are in the path of the flooding on the Russian side, according to a Ukrainian official.

In telephone interviews arranged by a group distributing humanitarian aid in Antonivka, residents described how they had watched as rising waters crept from house to house. They kept their distance from the river bank, where Russian snipers on the opposite side have in the past fired at residents, they said.

The eastern bank of the river, south of the dam, is controlled by Russian forces.

The damage threatens to disrupt vital services provided by the dam’s reservoir. It will cause a severe shortage of drinking water in the Dnipro, Zaporizhzhia and Kherson regions, Ihor Syrota, the hydropower company chief, said.

Flooding could also wash mines from their original positions into previously safe areas. And Russian officials says the destruction may pose problems for a canal supplying water to Crimea.

It also provides water for the cooling of reactors and spent fuel at the nearby Zaporizhzhia Nuclear Power Plant, but Ukrainian officials and the U.N.’s nuclear watchdog said Tuesday that the facility is not at immediate risk of meltdown as a result of the damage to the dam.

The six reactors at the Zaporizhzhia nuclear power plant are not the only sites that require cooling water, said Ivan Plachkov, a former energy minister of Ukraine said. Cooling pools for spent fuel and for new fuel waiting to be loaded into reactors require circulating water, he said. In total, the plant now requires between 130,000 to 260,000 gallons of water per hour, he said.

The cooling pools for spent fuel at the Zaporizhzhia nuclear power plant are a particular concern, Plachkov said. Reserve sources of water in cooling ponds exist on site, he said. But if water is not circulated in the spent fuel cooling pool, the pool could boil and quickly evaporate. Without water, the spent fuel will melt, he said.

All six nuclear reactors at the Zaporizhzhia Plant are shut down but still require water to dissipate heat from the radioactive fuel remaining in the reactor cores, Ivan Plachkov, a former minister of energy of Ukraine, said in an interview. The cooling systems can operate for some time on a closed loop, circulating water within the station rather than drawing cooling water from the reservoir, he said.

Still, Plachkov said, a complete loss of cooling water would not be safe for the plant: "It is a very dangerous situation.”

The U.N. nuclear watchdog, the International Atomic Energy Agency, said it was “closely monitoring” the situation surrounding the dam but that there was “no immediate nuclear safety risk” at the nearby Zaporizhzhia Nuclear Power Plant. The plant draws the water needed for cooling from the reservoir above the dam.

Energoatom, Ukraine’s state nuclear power company, said the destruction of the dam “may have negative consequences” for the plant but that it now had sufficient water in a pond for cooling.

The clip also depicted Mr. Putin declaring a general mobilization, saying all the power of the country needed to be harnessed to defeat a “dangerous and insidious enemy.”

The press service of Mir, a Russian public broadcaster, said in a statement released to the state news agency Tass that its radio and television channels had been illegally interrupted for a little more than a half-hour before being restored.

It was unclear who was behind the fake speech. Dmitri Peskov, a Kremlin spokesman, told Tass that Mr. Putin “definitely” did not record any such emergency address and that an investigation was underway into what he called a “hack.”

The broadcast — which appeared to piece together genuine recordings of Mr. Putin’s voice to create a realistic spoof — coincided with a surge in Ukrainian attacks along the front line that may signal the start of Kyiv’s long-awaited counteroffensive.

The fake address — which claimed that the Ukrainian military had invaded three regions of Russia, including Belgorod — came after a series of attacks on Belgorod by militias aligned with Ukraine. The attacks, which have been claimed by two paramilitary groups made up of Russians who oppose the Kremlin, have prompted evacuations in some areas on Russian soil.

Alina Lobzina contributed reporting.