An undated aerial handout photo shows the National Security Agency (NSA) headquarters building in Fort Meade, Maryland - Reuters

The US National Security Agency has for several years extensively targeted the communication systems of China and Hong Kong, according to the NSA Prism whistleblower Edward Snowden.

The former CIA employee, who is believed to be on a plane for Moscow after hiding out in Hong Kong, has said the US intelligence agency had launched several attacks aimed at Beijing's top Tsinghua University, which is well known for its research in China.

The university also houses one of the six "network backbones" which route the whole of China's internet traffic. The network, Beijing's first internet backbone, is under the Chinese ministry of education and is maintained by the university authorities.

Although it is not clear how many times NSA has resorted to tapping, documents shared by Snowden with the South China Morning Post indicate the last bout of hacking took place in January.

The university is also home to the Hong Kong Internet Exchange.

"The NSA does all kinds of things like hack Chinese cell phone companies to steal all of your SMS data," Snowden told the Chinese daily in an interview.

Earlier, he had said that the British agency, GCHQ, is worse than the NSA when it comes to cyber-snooping on worldwide communications.

Apart from mainland China, Hong Kong, a Chinese territory, has also been the target of lengthy cyber-attacks, Snowden said.

NSA experts repeatedly attempted to break into the networks of Pacnet, owner of one of the biggest fibre-optic networks in the Asia-Pacific region, he said.

The company, with headquarters in Hong Kong and Singapore, owns more than 46,000km of fibre-optic submarine cables.