Beijing on Thursday strongly denied a US congressional panel's claim that Chinese government-linked hackers had launched dozens of cyberattacks on American defence contractors, saying the charges were "fabricated out of thin air".

The denial came a day after a Senate panel released a study finding that hackers had gained access to systems run by companies doing contract work for the US Transportation Command (TRANSCOM) at least 50 times between 2012 and 2013, potentially compromising military operations.

"Chinese law prohibits cyberattacks and other crimes, and cracks down on such crimes," Chinese foreign ministry spokesman Hong Lei said. "The Chinese government and military will never support hacker attacks."

"The accusation against China is fabricated out of thin air, and is groundless," he added. "We urge the American side to stop its irresponsible accusations against China."

The row is the latest cyber-security spat to roil US-China relations.

In May this year, China banned the use of Microsoft's Windows 8 operating system on all new government computers, amid reports alleging security concerns.

That followed the United States indicting five members of a Chinese military unit for allegedly hacking US companies for trade secrets.

The New York Times in July quoted US officials saying Chinese hackers accessed US government computers containing personal information on all federal employees.

The Senate Armed Services Committee's report found that of the 50 attacks, at least 20 were "attributed to an 'advanced persistent threat' (APT), a term used to distinguish sophisticated cyber threats that are frequently associated with foreign governments".

The report, as well as committee chairman Senator Carl Levin, attributed all 20 APTs to China.

"The security of our military operations is what is at stake," Levin told reporters as he unveiled a de-classified version of the report.

The hacking has apparently occurred for years. Between 2008 and 2010, for example, a "Chinese military intrusion" into a TRANSCOM contractor compromised emails, documents, computer code and passwords, the report said.