China’s army may be behind a hacking group that has attacked at least 141 companies as well as government agencies worldwide since 2006, according to a report by a U.S. security firm.

The attacks, mainly directed at U.S. companies, were carried out by a group that is “likely government sponsored” and is similar “in its mission, capabilities, and resources” to a unit of the People’s Liberation Army, Mandiant Corp. said in a report Tuesday. Mandiant said it traced the group, labeled Advanced Persistent Threat 1, to four large computer networks in Shanghai.

China has frequently been accused of hacking, but the report contains some of the most extensive and detailed accusations to date.

Mandiant said it traced the hacking back to a neighborhood in the outskirts of Shanghai that includes a drab, white 12-story office building run by the secret “Unit 61398” of the People’s Liberation Army.

“It is time to acknowledge the threat is originating in China,” Alexandria, Virginia-based Mandiant said. “Our research and observations indicate that the Communist Party of China is tasking the Chinese People’s Liberation Army to commit systematic cyber espionage and data theft against organizations around the world.”

A recently prepared U.S. secret intelligence assessment, described Feb. 11 in the Washington Post, said the country’s economy is endangered by a massive and prolonged computer- espionage campaign from China. The New York Times also last month said its computer systems were breached by Chinese hackers, a claim China has denied.

‘Unfounded Accusations’

China’s Foreign Ministry said today the country opposes computer hacking and that it is a victim of attacks. The U.S. is the biggest attacker of China’s Internet, said Hong Lei, a spokesman for the ministry. Making “unfounded accusations” is not conducive to resolving the issue, Hong said when asked about the report at a regular briefing today.

“It’s inaccurate and unprofessional to accuse the Chinese military of Internet attacks,” the Ministry of Defense said by fax today in response to a Bloomberg News request for comment on the Mandiant report. “China’s military has never supported hacking and the country has always cracked down on relevant criminals.”

Bloomberg News reported in November that hackers from China broke into computer systems at Coca-Cola Co. in 2009 and reported in July that Chinese hackers targeted 20 organizations, including the European Union Council, while being monitored by security researchers in 2011.

Bloomberg’s Dune Lawrence, while reporting on Chinese online espionage this month, also suffered a hacker attack that Google Inc. said may have been state-sponsored.

APT1 has attacked companies in 20 major industries, and 87 percent of the targets are based in countries where English is the first language, according to the Mandiant report. Targets were based in countries including the U.S., Canada, the U.K., India and Singapore. Mandiant’s research was reported earlier by the New York Times.

“Sound” Investigation

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 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.”