The United States has filed espionage charges against Edward Snowden, a former US National Security Agency contractor who admitted revealing secret surveillance programmes to media outlets, according to a court document made public.

The charges are the US government's first step in what could be a long legal battle to return Mr Snowden from Hong Kong.

Mr Snowden is believed to be in hiding, and authorities want to try him in a US court.

A Hong Kong newspaper said he was under police protection, but the territory's authorities declined to comment.

Mr Snowden was charged with theft of government property and unauthorised communication of national defence information.

He is also charged with willful communication of classified communications intelligence to an unauthorised person, said the criminal complaint, which was dated 14 June.

The latter two offences fall under the US Espionage Act and carry penalties of fines and up to 10 years in prison.

Two US sources, speaking on condition of anonymity, said the US was preparing to seek Mr Snowden's extradition from Hong Kong, which is part of China but has wide-ranging autonomy, including an independent judiciary.

The Washington Post, which first reported the criminal complaint said the US had asked Hong Kong to detain Snowden on a provisional arrest warrant.

Hong Kong's Chinese-language Apple Daily quoted police sources as saying that anti-terrorism officers had contacted MR Snowden, arranged a safe house for him and provided protection.

The report said the police had checked his documents but had not discussed other matters or taken any statements.

Hong Kong Police Commissioner Andy Tsang declined to comment other than to say Hong Kong would deal with the case in accordance with the law.

Mr Snowden earlier this month admitted leaking secrets about classified US surveillance programmes, creating a public uproar.

Supporters say he is a whistleblower, while critics call him a criminal and perhaps even a traitor.

He disclosed documents detailing US telephone and Internet surveillance efforts to the Washington Post and Britain's Guardian newspaper.

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

That judicial district has seen a number of high-profile prosecutions, including the spy case against former FBI agent Robert Hanssen and the case of al Qaeda operative Zacarias Moussaoui.

Both were convicted.

Under a US programme called Prism the NSA has access to vast amounts of internet data.

Documents leaked by Mr Snowden revealed that access extends to  emails, chat rooms and video from large companies such as Facebook and Google.

The documents also showed that the government had worked through the secret Foreign Intelligence Surveillance Court to gather so-called metadata .

Phone information such as the time, duration and telephone numbers called has also been accessed.