Edward Snowden, who has been charged with espionage after leaking details of secret US spy programs has left Hong Kong, believed to be heading for an unnamed "democratic nation" via Moscow.

The 30-year-old former CIA contractor has been holed up in Hong Kong since blowing the lid on America's extensive surveillance operations.

"US whistleblower Edward Snowden has left Hong Kong on an Aeroflot flight to Moscow, credible sources have confirmed to the South China Morning Post," the newspaper, which has carried exclusive interviews with Snowden during his time in Hong Kong, said on its website.

"Moscow will not be his final destination."

Russia's Interfax news agency is reporting a source at Aeroflot airline saying there is a ticket in Snowden's name for a flight from Moscow to Cuba before taking a local flight to the Venezuelan capital Caracas.

Wikileaks has released a statement on its website claiming to have helped Snowden leave Hong Kong and find "political asylum in a democratic country".

"Mr Snowden requested that WikiLeaks use its legal expertise and experience to secure his safety," Wikileaks says.

The United States had been seeking to extradite Snowden from Hong Kong to try him over espionage charges.

A Hong Kong government spokesman has confirmed Snowden left the territory, saying he did so voluntarily and through "legal" means.

"Snowden today voluntarily left Hong Kong for a third country through legal and normal means," a Hong Kong government spokesman said in a press statement.

The spokesman added that Hong Kong had "not obtained adequate information" to handle a provisional arrest warrant for Snowden issued by the US.

"As the HKSAR government has yet to have sufficient information to process the request for provisional warrant of arrest, there is no legal basis to restrict Mr Snowden from leaving Hong Kong," it said.

US vows to continue pursuing Snowden

The US Justice Department says it will seek the cooperation of law enforcement authorities in countries where Snowden may travel.

"We will continue to discuss this matter with Hong Kong and pursue relevant law enforcement cooperation with other countries where Mr Snowden may be attempting to travel," spokeswoman Nanda Chitre said in a statement.

House Intelligence Committee chairman Mike Rogers says the US government must exhaust all legal options to get Snowden back to the US because the leaking of secret government surveillance programs had damaged national security.

Meanwhile, National Security Agency director Keith Alexander said it is overhauling its operations to keep a closer watch on contractors like the fugitive Snowden.

"Clearly, the system did not work as it should have," Mr Alexander said.

"We are now putting in place actions that would give us the ability to track our system administrators, what they doing, what they are taking."

Earlier, the South China Morning Post quoted Snowden offering new details about America's spy activities, including accusations of US hacking of Chinese mobile phone companies and targeting China's Tsinghua University.

Following the revelations, China's official news agency labelled the US as the world's biggest espionage "villain" and called for an explanation.

Snowden's first leaks revealed that the US National Security Agency (NSA) has access to vast amounts of internet data such as emails, chat rooms and video under a government program known as PRISM.

Snowden's supporters say he is a whistleblower, while critics call him a criminal and perhaps even a traitor.

President Barack Obama and his intelligence chiefs have been forced to defend the PRISM program, saying it is regulated by law and that Congress was notified.

They say the program has been used to thwart militant plots and does not target Americans' personal lives.

ABC/wires