Although the PLA's internal computer network is physically isolated from other networks, new technical tools have allowed spies to wirelessly sneak into an electronic device or facility.

"Technically speaking, there is always the possibility of hacking a network from thousands of miles away if it is connected to the Internet, whether or not it has a firewall," Senior Colonel Tan said.

"The alleged Chinese military-backed hacking of US sites proves that they did not understand the PLA's functions and missions," said Tan, who specializes in military equipment and logistics.

Tan revealed that so far the PLA has not constituted a cyber warfare unit, saying the allegation was mystifying and merely an excuse for the US to strengthen cyber warfare technologies.

"Scenarios of different versions of Chinese hacking have not been rare, since the US is developing its own cyber warfare powers and overestimated the abilities of Chinese Internet users," he said.

Last year, accusations of Chinese hacking into the US Department of Defense network emerged in the US media just before the Pentagon announced the establishment of the new US Cyber Command, which is subordinated to the US Strategic Command and responsible for coordinating computer-network defense and cyber-attack operation.

"The PLA has academic researchers on information warfare, but is not capable of conducting actual cyber-attack operations. Chinese laws prohibit any forms of cyber-attack.

"Nor is the PLA allowed to hire civilian hackers, and hacking foreign government and company networks has nothing to do with the PLA's missions." Tan said.

Civilian networks in China seemed much more vulnerable to hackers. As a result, the Ministry of Industry and Information Technology Monday issued a nationwide plan to safeguard the security of domain name systems for government websites and vital networks amid surging Internet security threats and risks.

Last week, police in central China's Hubei Province destroyed the country's biggest hacker training organization and arrested three people who were suspected of running the Black Hawk Safety Network.

The network was suspected of offering online hacker tools, a crime that was listed in China's Criminal Law last year.

Statistics from the National Computer Network Emergency Response Technical Team/Coordination Center (CNCERT/CC) reveal about 262,000 Chinese computers were hijacked by Trojan programs tracked to overseas IP addresses in 2009. The top source of the programs, 16.61 percent, were computers based in the United States.

The number of Chinese computers controlled by botnets in 2009 was 837,000. A total of 19,000 overseas-hosted addresses, of which, 22.34 percent were from the US, participated in controlling the Chinese computers.