The US pressured Hong Kong to act quickly on its request to extradite Edward Snowden, a former US National Security Agency contractor charged with espionage for exposing secret US surveillance activities.

"If Hong Kong doesn't act soon, it will complicate our bilateral relations and raise questions about Hong Kong's commitment to the rule of law," a senior Obama administration official told Reuters on Saturday, speaking on condition of anonymity.

Sources say Snowden, who has been hiding in Hong Kong, has sought legal representation from human rights lawyers as he prepares to fight attempts to force him back to the US to face trial.

US National Security Adviser Tom Donilon told CBS News the US had a "good case" against Snowden and expected Hong Kong to comply with its 1998 extradition treaty with the US.

"We have gone to the Hong Kong authorities seeking extradition of Snowden back to the United States," Donilon said.

He added that US law enforcement officials were in a "conversation" with Hong Kong authorities about the issue.

"Hong Kong has been a historically good partner of the United States in law enforcement matters and we expect them to comply with the treaty in this case," Donilon said.

Arrest warrant

US authorities had earlier issued a provisional arrest warrant and filed espionage and other charges against Snowden.

Confirming a report in the Washington Post newspaper, a US official said that a sealed criminal complaint had been lodged with a federal court in the US state of Virginia and a provisional arrest warrant had also been issued.

Snowden was charged with theft of government property, unauthorised communication of national defence information and willful communication of classified communications intelligence information to an unauthorised person, the document said.

Snowden is reported to be in hiding in Hong Kong, and the US has also asked Hong Kong to detain Snowden on the provisional arrest warrant, the Washington Post newspaper reported, citing unnamed US officials.

The criminal complaint was filed in the Eastern District of Virginia, where Snowden's former employer, Booz Allen Hamilton, is located, the Post reported.

Troves of documents leaked by Snowden revealed that US security services had monitored data about phone calls from Verizon and Internet data from large companies such as Google and Facebook as part of counterterrorism efforts.

Meanwhile, an Icelandic businessman linked to the anti-secrecy group WikiLeaks said on Thursday he had readied a private plane in China to fly Snowden to Iceland if Iceland's government would grant asylum.

Iceland refused on Friday to say whether it would grant asylum to Snowden.

Source: Agencies