The sites were subject to about 144,000 hacking attacks each month last year, two thirds of which came from the US, according to China's defence ministry.
The issue of cyber hacking has strained relations between the two countries.
Earlier this month a US cyber security firm said a secretive Chinese military unit was behind "prolific hacking".
Mandiant said that Unit 61398 was believed to have "systematically stolen hundreds of terabytes of data" from at least 141 organisations around the world.
The White House has said that it has taken its concerns about cyber-theft to the highest levels of China's government. China denied the allegations,  saying it was also the victim of cyber attacks.
The US is yet to respond to these latest allegations from China.
"The Defence Ministry and China Military Online websites have faced a serious threat from hacking attacks since they were established," defence ministry spokesman Geng Yansheng is quoted by Reuters news agency as saying at a monthly press conference.
He added that the number of attacks on these sites  had steadily increased over the years.
An analysis of the IP addresses involved showed that officials had ascertained that attacks from the US accounted for 62.9 percent of the attacks made on these two website in 2012, according to Mr Geng.
He also said that reported US plans to expand its cyber warfare capabilities were unlikely to foster international collaboration.
"We hope that the U.S. side can explain and clarify this," he said.
It is believed to be the first time that Chinese officials have provided such details about alleged US-based attacks on their own systems.
However, Beijing has been accused by several governments, foreign companies and organisations of carrying out extensive cyber espionage for many years, seeking to gather information and to control China's image.
In late January, the New York Times said that hackers from China had "persistently" infiltrated the paper over the previous four months, saying the attacks had coincided with its reports into the wealth of the family of Chinese Premier Wen Jiabao.
At the time China's foreign ministry dismissed those accusations as "groundless".
Hackers from the US have repeatedly launched attacks on two Chinese military websites, including that of the Defence Ministry, officials say.