Hackers mainly based in the United States attacked two Chinese military websites including the Defence Ministry page an average of 144,000 times a month last year, the ministry said on Thursday.

China's first report of attacks on its websites steps up a war of words between the powers, after a US security company said last week that a Chinese military unit was behind a series of hacking attacks on US firms.

"The Defence Ministry and China Military Online websites were hacked from overseas on average 144,000 times a month in 2012," ministry spokesman Geng Yansheng said on the ministry's website.

China Military Online is a People's Liberation Army news website.

Some 62 percent of the attacks came from the United States, he said, adding that the number of hacking assaults on military websites "has risen steadily in recent years".

He did not specify any entities from which the alleged attacks originated.

A report from US security firm Mandiant said a unit of China's People's Liberation Army had stolen hundreds of terabytes of data from at least 141 organisations, mostly based in the United States.

China's defence ministry had said the report had "no factual basis".

Geng called on US officials to "explain and clarify" what he said were recent US media reports that Washington would carry out "pre-emptive" cyber attacks and expand its online warfare capabilities.

Such efforts are "not conducive to the joint efforts of the international community to enhance network security", he said.

Geng also said that while China's military forces were working hard to push ahead with what he called "informatisation", they still had some distance to go.

"There is still a certain gap between the building up of China's military informatisation and the advanced global military level," he said. "At present, China's military has no cyber warfare units."

Hacking accusations have strained ties between Washington and Beijing, with State Department spokeswoman Victoria Nuland saying this month that hacking comes up "in virtually every meeting we have with Chinese officials".

Last month the New York Times and other American media outlets reported they had come under hacking attacks from China, and a US congressional report last year named the country as "the most threatening actor in cyberspace".

China has called the charges groundless and state media have accused Washington of making China a scapegoat to deflect attention from US economic problems.