BEIJING - American cyber-security experts failed to provide sufficient evidence when accusing Chinese cyber spies of trying to break into computers belonging to China specialists and defense contractors in the United States, a Chinese cyber expert told Xinhua on Thursday.

These American experts did not ground such an accusation with technically convincing evidence but only on the resemblance of this incident to previous ones in tactics, which was rather reckless, said Dai Yiqi, a cyber defense expert from School of Computer Science of Tsinghua University.

On June 5, the Wall Street Journal cited American cyber security experts as saying, that Chinese cyber spies were attempting to break into the computers of China experts and defense contractors in the United States who frequent the American government.

This is a second accusation lodged against Chinese hackers after Google blamed them on June 1 for hacking into the Gmail accounts of American senior government officials and military personnel.

According to the Wall Street Journal, China specialists were tricked into opening attachments that would provide hackers access to their computers, and the move was similar in tactics to what Google disclosed.

Besides, James Mulvenon, a China and cyber-security expert in the United States, was quoted by the Wall Street Journal as saying that these emails contained many spelling mistakes and odd wording choices that made more sense in Chinese than American English.

Dai said this evidence was insufficient and cannot support their arguments that the source of attack came from China. Even if the source of the attack was spotted, it was not necessarily the true base of attackers.

They were likely to identify the wrong source, given the complex structure and numerous nodes on the Internet, said Dai.

Such an ungrounded accusation also violates the principle that "the proof lies upon the one who affirms" in British and American legal traditions, said Ding Xiangshun, an associate professor from the Law School of Renmin University of China.

"The burden of proof lies upon him who affirms, not him who denies. Therefore, if these people accuse Chinese hackers for the attack, they are supposed to provide sufficient proof, rather than ask the accused to prove themselves innocent," Ding said.

Shi Yinhong, a professor in International Relations with Renmin University, said accusations of Chinese hackers from the United States in recent years were mostly generated out of thin air, and their true intent was to point finger at the Chinese government.

However, China is the hardest-hit of cyber attacks. In 2010, host computers of over 4.51 million IP addresses inside China were implanted with Trojan horses, up 1620.3 percent from 2009, according to National Computer Network Emergency Response Technical Team/Coordination Center.

In 2010, IP addresses of servers outside China controlling Trojans amounted to about 220,000, up 57 percent from 2009. The largest share of the servers, 14.66 percent, came from the United States, also according to the center.