Former spy Edward Snowden has classified US documents which show the specific machines targeted by the National Security Agency in China and Hong Kong, according to a report Friday.

The South China Morning Post said a small sample of records had been shown to the newspaper during an interview with Snowden which detailed the Internet Protocol (IP) addresses of targets and dates of cyber attacks.

IP addresses are unique numbers assigned to individual computers and other devices attached to the Internet. The SCMP did not publish the addresses themselves or identify individuals who might have been targeted.

The latest report came after the FBI said Thursday that the United States had launched a criminal investigation and is taking "all necessary steps" to prosecute Snowden for exposing state secrets.

The documents seen by the SCMP, but which could not be verified, also showed details of whether a cyber attack was ongoing, the paper said, and appeared to suggest a hacking success rate of 75 percent.

"I don't know what specific information they were looking for on these machines, only that using technical exploits to gain unauthorised access to civilian machines is a violation of law. It's ethically dubious," 29-year-old Snowden told the newspaper.

The revelation that Snowden possesses specific details of individuals targeted will pile extra pressure on the US to stem the leaks.

Snowden, who worked as a subcontractor handling computer networks, is in hiding in Hong Kong after arriving in the city on May 20 and leaking details of a global eavesdropping operation by the NSA.

On Wednesday he vowed to remain in the city and fight in court any efforts by the US to extradite him.

FBI chief Robert Mueller has defended the Internet and phone sweeps as a vital tool in the US fight against terror and said they were implemented legally in accordance with the Constitution.