Suspected Chinese hackers tried to steal the passwords of hundreds of Google email account holders - including those of senior US government officials, Chinese activists and journalists - the Internet company said.
The claim by the world's largest Web search engine sparked an angry response from Beijing, which said blaming China was "unacceptable", pointing to further tensions in an already strained relationship with Google.
The perpetrators appeared to originate from Jinan, the capital of China's eastern Shandong province, Google said. Jinan is home to one of six technical reconnaissance bureaus belonging to the People's Liberation Army and a technical college US investigators last year linked to a previous attack on Google.
Washington said it was investigating Google's claims, while the FBI said it was working with Google following the attacks - the latest computer-based invasions directed at multinational companies that have raised global alarm about Internet security.
The hackers recently tried to crack and monitor email accounts by stealing passwords, but Google detected and "disrupted" their campaign, the company said on its official blog. Google said it had notified the victims.
The revelation comes more than a year after Google disclosed a cyberattack on its systems that it said it traced to China. Google partially pulled out of China, the world's largest Internet market by users, last year after a tussle with the government over censorship.
"We recently uncovered a campaign to collect user passwords, likely through phishing," Google said, referring to the practice where computer users are tricked into giving up sensitive information.
It "affected what seem to be the personal Gmail accounts of hundreds of users, including among others, senior US government officials, Chinese political activists, officials in several Asian countries (predominantly South Korea), military personnel and journalists."
A Washington-based security expert, Mila Parkour, first reported the Gmail attacks on her blog in February, saying they appeared to have started last year and were invasive.
China's Foreign Ministry said it "cannot accept" accusations hackers in China tried to break into hundreds of Gmail accounts. Google did not say the Chinese government was behind the attacks or say what might have motivated them.
But a former US government official who served in China said he was fairly sure the Chinese government was responsible. He said it was a sign of Beijing's fears that contagion from the Arab "jasmine" uprisings could spread to China.
The United States has warned that a cyberattack - presumably if it is devastating enough - could result in real-world military retaliation, although analysts say it could be difficult to detect its origin with full accuracy.
"We have no reason to believe that any official US government email accounts were accessed," said White House spokesman Tommy Vietor.
Lockheed Martin, the US government's top information technology provider, said last week it had thwarted "a significant and tenacious attack" on its information systems network, though the company and government officials have not yet said where they think the attack originated.
Reuters