The United States government is hacking Chinese mobile phone companies to gather data from millions of text messages, former intelligence technician Edward Snowden told the Sunday Morning Post in Hong Kong.

US spies have also hacked China's prestigious Tsinghua University in Beijing and Asia Pacific fibre-optic network operator Pacnet, the Post quoted Snowden as saying.

Snowden, who worked as a contractor for the National Security Agency (NSA), has been charged with espionage by the US after revealing a massive spying programme and has gone to ground after fleeing to Hong Kong.

"The NSA does all kinds of things like hack Chinese cell phone companies to steal all of your SMS data," Snowden said in the interview conducted on June 12.

The Post said Snowden had provided documents listing operational details of specific attacks on computers, including internet protocal (IP) addresses, over a four-year period.

Government data shows almost 900 billion text messages were exchanged in China in 2012.

The claims followed soon after a report in the Guardian in which he claimed the British government's electronic eavesdropping agency had gained secret access to fibre-optic cables carrying global Internet traffic and telephone calls.

Britain's Guardian said that Government Communications Headquarters (GCHQ) had started processing vast amounts of personal information -- including Facebook posts, emails, Internet histories and phonecalls -- and is sharing it with the NSA.

The Post has previously quoted Snowden saying there have been more than 61,000 NSA hacking operations globally, targeting powerful "network backbones" that can yield access to hundreds of thousands of individual computers.

He said these included hundreds of targets in mainland China and Hong Kong.

Snowden told the Post in the report published Saturday that Tsinghua University, which counts China's President Xi Jinping and previous President Hu Jintao among its graduates, was the target of extensive hacking by the US.

The university, which is home to one of the mainland's six major backbone networks from where Internet data from millions of Chinese citizens can be gathered, was breached as recently as January, he said.

In 2009, the NSA also attacked Pacnet, the owner of one of the region's biggest fibre-optic networks, the Post reported, citing information provided by Snowden.

Pacnet, which is headquartered in Hong Kong and Singapore, owns 46,000 kilometres of fibre and operates in 13 countries, according to its website.

A US justice department official has confirmed that a sealed criminal complaint has been lodged with a federal court in Virginia and a provisional arrest warrant has been issued for Snowden, who fled to Hong Kong in May.

But Hong Kong government officials Saturday remained tight-lipped as to whether they had received such a request and whether Snowden had been approached.

"If jurisdictions which have signed an agreement with Hong Kong made a request, then the Hong Kong government would process any request in accordance. And we would deal with it according to the law and system of Hong Kong," Police Commissioner Andy Tsang told reporters.

The Post claimed on its website late Saturday that Snowden remained "safe" in Hong Kong and had not been detained by police.

The city of seven million people has maintained a degree of autonomy since its handover of sovereignty from Britain to China in 1997 and operates a separate legal system.

The former British colony has a long-standing extradition treaty with the US, but Beijing, which has control over the city's defence and foreign affairs, has the potential to veto any ruling.

Legal experts have said that any attempt to extradite Snowden could last several years.