China's Defense Ministry did not immediately respond to faxed questions about the report, although it has in the past labeled such allegations as groundless and irresponsible, and has demanded that evidence be presented.
News of the report spread Tuesday on the Chinese Internet, with many commentators calling it an excuse for the U.S. to impose greater restrictions to contain China's growing technological prowess.
Graham Cluley, a British cybersecurity expert who was not involved in Mandiant's research, said people in the computer industry believe China's government is behind such attacks but have been unable to confirm the source.
"None of us would be very surprised or be uncomfortable saying we strongly suspect the Chinese authorities are involved in spying this way," said Cluley, a senior technology consultant for security firm Sophos in Britain.
"I think we are seeing a steady escalation" of sophistication in hacking, Cluley said. "This is really the new era of cybercrime. We've moved from kids in their bedroom and financially motivated crime to state-sponsored cybercrime, which is interested in stealing secrets and getting military or commercial advantage."
Associated Press writers Gillian Wong and Joe McDonald contributed to this report.
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