Online attackers linked to China are behind a vast cyber espionage campaign targeting government agencies, Google subsidiary Mandiant said on Thursday.

The cyber espionage activity was detected in May and is believed to have started last October. The hackers targeted victims in at least 16 different countries, striking organizations in the public and private sectors worldwide, the report said.

"This is the broadest cyber espionage campaign known to be conducted by a China-nexus threat actor since the mass exploitation of Microsoft Exchange in early 2021," said Mandiant chief technology officer Charles Carmakal.

Mandiant reported having "high confidence" that a group referred to as UNC4841 was behind the wide-ranging espionage campaign "in support of the People's Republic of China." The cyberattackers compromised the computer defenses of hundreds of organizations, in some cases stealing "emails of prominent employees dealing in matters of interest to the Chinese government," Carmakal added.

The targeting focused on issues of high policy importance to the Chinese government, particularly in the Asia-Pacific region and Taiwan, according to the report. Victims included foreign ministries, research organizations, and foreign trade missions based in Hong Kong and Taiwan, Mandiant said in its findings.

The attacks involved email messages booby-trapped with malicious code and exploited a vulnerability in Barracuda software for screening such missives to make sure they are safe, according to the report.

The 2021 hack of Microsoft Exchange, which security researchers attributed to a Beijing-backed group, affected at least 30,000 organizations in the United States including businesses and local governments. Washington has frequently accused Beijing of cyberattacks against U.S. targets, with the issue contributing to a deterioration in relations between the two powers in recent years.