American officials are claiming that the Chinese military successfully hacked computers inside the Pentagon in June.
The officials said that they had a "very high level of confidence ... trending towards total certainty" that the attacks came from the People's Liberation Army [PLA]".
"The PLA has demonstrated the ability to conduct attacks that disable our system ... and the ability in a conflict situation to re-enter and disrupt on a very large scale," said a former Pentagon official.
The attack caused the Pentagon to take down 1,500 computers for checking and cleaning.
China has denied claims that it was involved.
Meanwhile, German chancellor Angela Merkel has complained that German systems have been penetrated by Chinese hackers. She raised her complaints with Hu Jintao, China's president.
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