The US firm Mandiant said cyberattacks had been traced back to a non-descript, 12-story building on the outskirts of Shanghai, where China's army was believed to be in charge of hundreds if not thousands of hackers.

In a 74-page report, the firm said that the hacking group "APT1" -- from the initials "Advanced Persistent Threat" -- was believed to be a branch of what is known as Unit 61398 of the People's Liberation Army.

"We believe that APT1 is able to wage such a long-running and extensive cyber espionage campaign in large part because it receives direct government support," Mandiant said.

US officials voiced concern about the charges in the report but were cautious. White House spokesman Jay Carney said US officials "regularly raise this issue with Chinese officials, including officials in the military."

And State Department spokeswoman Victoria Nuland said cybertheft was a "serious concern" that comes up "in virtually every meeting we have with Chinese officials" including "at the highest levels."

"We consider this kind of activity a threat not only to our national security but also to our economic interests and (we are) laying out our concerns specifically so that we can see if there's a path forward," she said.

Senator Dianne Feinstein, head of the intelligence committee, said that the Mandiant study showed the need for an international agreement on cyberattacks along the lines of codes that cover crime and war.

"Absent that, the United States remains especially vulnerable to a potential catastrophic cyberattack," Feinstein, a Democrat from California, said in a statement.

Feinstein voiced alarm that "there is nothing currently in place to govern this emerging and increasingly dangerous national and economic security threat."

Earlier, China's defense ministry said its army had never supported any kind of hacking activity.

"Not only are reports that China's army has been involved in hacking unprofessional, they do not fit with the facts," the ministry said in a statement to AFP.

"Hacking attacks are a global problem. Like other countries, China also faces the threat of hacking attacks, and is one of the main countries falling victim to hacking attacks."

The country's foreign ministry rejected "groundless accusations" of Chinese involvement in hacking and also said Beijing was itself a major victim, with most overseas cyberattacks against it originating in the United States.

The United States has been stepping up its cyber warfare capabilities. America and Israel are widely believed to have unleashed the Stuxnet virus several years ago in a bid to cripple Iran's contested nuclear program.

But concerns have been rising in the United States as prominent companies and media outlets report cyberattacks. Apple on Tuesday was the latest to report a hacking attempt, although it said no data was stolen.

The New York Times said hackers stole its corporate passwords and accessed the personal computers of 53 employees after the newspaper published a report on the family fortune of China's Premier Wen Jiabao.

In its report, Mandiant alleged that APT1 -- known also as "Comment Crew" for its practice of planting viruses on the comment sections of websites -- has stolen hundreds of terabytes of data from at least 141 organizations spanning 20 industries.

The New York Times, which was given early access to the report, said the researchers had found that the Comment Crew was increasingly focused on companies involved in US infrastructure, including in its electrical power grid, gas lines and water works.

One target, the newspaper reported, was a company with remote access to more than 60 percent of oil and gas pipelines in North America.

In his State of the Union address last week, US President Barack Obama said the potential ability of outsiders to sabotage the nation's critical infrastructure was a major concern.

"We cannot look back years from now and wonder why we did nothing in the face of real threats to our security and our economy," he said.