China on Friday rejected a US accusation that it uses cyber espionage to steal sensitive economic information and technology from the United States, calling the claim "unprofessional and irresponsible".

In?an?unusually blunt report submitted to the US Congress on Thursday,?China was accused of the?world's "most active and persistent perpetrators" of economic espionage, according to AFP.

It also claimed China and Russia had launched an "onslaught" of cyber espionage on US companies to win bargaining power and trade secrets.

Chinese Foreign Ministry spokesman Hong Lei rejected the accusation at a regular news briefing on Friday, and repeated Beijing's long-standing position that it wants to help fight cyber espionage.

"Online attacks are notable for spanning national borders and being anonymous. Identifying the attackers without carrying out a comprehensive investigation and making inferences about the attackers is both unprofessional and irresponsible," Hong said.

"I hope the international community can abandon prejudice and work hard with China to maintain online security," he added.

The report acknowledged the difficulty of determining who exactly is behind a cyber attack.

China has repeatedly refuted accusations that it is involved in cyber spying. At a regular news conference on Wednesday, Hong reiterated Beijing's insistence that it also has been attacked.

"China is a major victim of hacking," Hong said. "China is ready to build, together with other countries, a peaceful, secure and open cyberspace order."

On Monday, China also rejected a US accusation that it had hacked into an American satellite system.

"What we should care about is whether there is any suggested punitive measure in (Thursday's) report, which could be really consequential for Sino-US relations," said Sun Zhe, a professor of the Institute of International Studies with Tsinghua University.

"If Americans really want their accusation to be valid, they should provide evidence to prove (their claims) and whether it is backed by government," Sun said.

In August, Jim Lewis, a cyber-security expert with the US Center for Strategic and International Studies, told US media that Russia and China could be behind an unprecedented series of cyber attacks.

But Dmitri Alperovitch, McAfee's vice-president of threat research, said they "don't have direct evidence that conclusively points to a particular nation state", according to Xinhua News Agency.

Wen Weiping, a cyber expert with the School of Software and Microelectronics of Peking University, also said the lack of evidence is the main problem.

"Technically speaking, the origin of the cyber hacking can be traced back to an IP address. Even though that address is not necessarily the real origin, it helps to determine the cause of the hacking," Wen said. "But we didn't see the US sharing such information."