China's first account 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 Defense 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 its website.
China Military Online is a People's Liberation Army news website.
More than 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 said in an earlier statement that 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 capabilties.
Such efforts are "not conducive to the joint efforts of the international community to enhance network security", he said.
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".