Reiterating a standard China government response on hacking claims, Hong said China itself is a major victim of such crimes, including attacks originating in the United States.
"As of now, the cyberattacks and cybercrimes China has suffered are rising rapidly every year," Hong said.
Mandiant's methodology used in the investigation was sound, said Massimo Cotrozzi, managing director of KCS Group, a London-based international cyber investigation consulting firm that was not involved in Mandiant's research.
"No one as yet has provided the world conclusive evidence of a link between the Chinese military and the attacks. This report is the nearest thing to conclusive evidence that I have seen," Cotrozzi said.
Mandiant said its findings led it to alter the conclusion of a 2010 report it wrote on Chinese hacking, in which it said it was not possible to determine the extent of government knowledge of such activities.
"The details we have analyzed during hundreds of investigations convince us that the groups conducting these activities are based primarily in China and that the Chinese government is aware of them," the company said in a summary of its latest report.
It said the hacking was traced to the 2nd Bureau of the People's Liberation Army General Staff's 3rd Department, most commonly known as unit 61398, in the Shanghai suburbs.
News of the report spread today 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.
The building housing "Unit 61398" of the People's Liberation Army is seen in the outskirts of Shanghai today. Cyberattacks that stole information from 141 targets in the U.S. and other countries have been traced to the Chinese military unit in the building, a U.S. security firm alleged today.