Separately, Chinese Foreign Ministry spokeswoman Hua Chunying called the Pentagon report on the Chinese military, an annual document submitted to Congress, an unjust criticism of China's justified and normal military development.
The Pentagon report said China has stepped up its military modernization program and is building space and cyberspace capabilities.
David Helvey, deputy assistant secretary of defense for East Asia, told reporters this year's report showed China is increasing its military modernization program under the country's new leadership. He said the integration and overlapping nature of China's weapons systems into a regime can potentially impede or restrict free military operations in the Western Pacific.
"In addition, in 2012, numerous computer systems around the world, including those owned by the United States government, continued to be targeted for intrusions, some of which appear to be attributable directly to [Chinese] government and military organizations," he said.
Chinese military experts said the cyberattack charges are groundless and irresponsible, the official Xinhua News Agency reported.
Wang Xinjun, researcher at the Chinese army's Academy of Military Sciences, said the Chinese government and armed forces have never sanctioned hacking activities, Xinhua reported.
He said both China and the United States are victims of cybercrimes and should work together to tackle the issue.
In her comment, Foreign Ministry spokeswoman Hua said: "We firmly oppose (the Pentagon) report and have made representations to the United States.
"China's necessary and moderate military buildup, which meets the country's needs, is completely aimed at safeguarding the country's independence, sovereignty and territorial integrity and part of the country's justified rights," Hua added.
In a separate article, Xinhua said the Pentagon report's reference to China's growing assertiveness of maritime territorial claims could "embolden countries engaging in maritime disputes with China, sowing instability in the Asia-Pacific region."