Image caption Edward Snowden fled to Hong Kong before the scandal broke

The US justice department has filed criminal charges against a fugitive ex-intelligence analyst who leaked details of a secret surveillance operation.

The charges against ex-National Security Agency (NSA) analyst Edward Snowden include espionage and theft of government property.

In May, Mr Snowden fled to Hong Kong after leaking details of a programme to monitor phone and internet data.

The US is also reported to be preparing an extradition request.

His leaks revealed that US agencies had systematically gathered vast amounts of phone and web data.

The criminal complaint was lodged with a federal court in the Eastern District of Virginia, court documents show, and a provisional arrest warrant had been issued, officials said.

Who is Edward Snowden? Age 30, grew up in North Carolina

Joined army reserves in 2004, discharged four months later, says the Guardian

First job at National Security Agency was as security guard

Worked on IT security at the CIA

Left CIA in 2009 for contract work at NSA for various firms including Booz Allen

Called himself Verax, Latin for "speaking the truth", in exchanges with the Washington Post Profile: Edward Snowden

Mr Snowden was charged with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence.

Each of the charges carries a maximum 10-year prison sentence. The complaint is dated 14 June although it was made public only on Friday.

Mr Snowden left a hotel in Hong Kong on 10 June after allowing newspapers to name him as the source of the leaks. His current whereabouts are unknown.

In the US, the charges were welcomed by Democratic Senator Bill Nelson, a member of the Senate Armed Services Committee.

"I've always thought this was a treasonous act,'' he said in a statement. "I hope Hong Kong's government will take him into custody and extradite him to the US."

However, some Hong Kong politicians have voiced support for Mr Snowden.

Left-wing MP Leung Kwok-hung said Beijing should tell the authorities to protect him from extradition, and the people of the territory should "take to the streets" to shelter him.

The BBC's Juliana Liu in Hong Kong says Beijing is highly unlikely to interfere in the early stages of what could be a long legal battle.

Julian Assange, founder of the Wikileaks organisation, also issued a statement supporting Mr Snowden.

"The US government is spying on each and every one of us, but it is Edward Snowden who is charged with espionage for tipping us off," said Mr Assange, who has been living in Ecuador's London embassy for the past year, fighting extradition to Sweden for questioning on alleged sex offences.

Beijing influence

Hong Kong, a Special Administrative Region of China, signed an extradition treaty with the US in 1998.

It has a separate legal system from the Chinese mainland, and Mr Snowden's right of appeal could drag out any future extradition proceedings for several years.

In a news conference on Saturday, Hong Kong police declined to comment on a local newspaper report that he is staying in a police safe house.

"According to Hong Kong's current system, if a jurisdiction that has signed the mutual legal assistance treaty has issued a request, then the Hong Kong government will treat it in accordance with current Hong Kong laws and systems," said police commissioner Andy Tsang.

The leaks led to revelations that the US is systematically seizing vast amounts of phone and web data under an NSA programme known as Prism.

Mr Snowden also alleged that US intelligence had been hacking into Chinese computer networks.

He said he had decided to speak out after observing "a continuing litany of lies" from senior officials to Congress.

US officials have since defended the practice of gathering telephone and internet data from private users around the world.

They say Prism cannot be used to intentionally target any Americans or anyone in the US, and that it is supervised by judges.

Earlier this week the head of the NSA, Gen Keith Alexander, told Congress that it had helped to thwart terror attacks.

In another development, the Guardian newspaper has reported that the UK is tapping fibre-optic cables and gathering large quantities of data.

The Guardian says its report is based on more documents released by Mr Snowden and leaked from the UK's electronic eavesdropping agency, GCHQ.

Data from global communications, including internet and phone use, was stored for up to 30 days to be analysed the papers says.