US National Security Agency whistleblower Edward Snowden, an analyst with a US defence contractor, is seen in this still image taken from a video during an interview with the Guardian in his hotel room in Hong Kong June 6, 2013. The 29-year-old contractor at the NSA revealed top secret U.S. surveillance programmes to alert the public of what is being done in their name, the Guardian newspaper reported on Sunday. [Photo/Agencies]

WASHINGTON  - An ex-CIA employee working as a contractor at the US National Security Agency said on Sunday he was the source who leaked details of a top secret US surveillance program, acting out of conscience to protect "basic liberties for people around the world."

Holed up in a hotel room in Hong Kong, Edward Snowden, 29, said he had thought long and hard before publicizing details of an NSA program code-named PRISM, saying he had done so because he felt the United States was building an unaccountable and secret espionage machine that spied on every American.

Snowden, a former technical assistant at the CIA, said he had been working at the super-secret NSA as an employee of contractor Booz Allen. He said he decided to leak information after becoming disenchanted with President Barack Obama, who he said had continued the policies of predecessor George W. Bush.

"I don't want to live in a society that does these sort of things ... I do not want to live in a world where everything I do and say is recorded. That is not something I am willing to support or live under," he told the Guardian newspaper, which published a video interview with him on its website.

Both the Guardian and the Washington Post said last week that US security services had monitored data about phone calls from Verizon and Internet data from large companies such as Google and Facebook.

In naming Snowden on Sunday, the newspapers said he had sought to be identified.  

"The NSA has built an infrastructure that allows it to intercept almost everything," Snowden said in explaining his actions.

"With this capability, the vast majority of human communications are automatically ingested without targeting. If I wanted to see your emails or your wife's phone, all I have to do is use intercepts. I can get your emails, passwords, phone records, credit cards," he said.

WORKED AT NSA FOR FOUR YEARS

The Guardian said Snowden had been working at the NSA for four years as a contractor for outside companies.

Three weeks ago, he copied the secret documents at the NSA office in Hawaii and told his supervisor he needed "a couple of weeks" off for treatment for epilepsy, the paper said. On May 20 he flew to Hong Kong.

The CIA and the White House declined to comment, while a spokesman for the Director of National Intelligence would not comment directly about Snowden himself but said the intelligence community was reviewing damage done by the recent leaks.

"Any person who has a security clearance knows that he or she has an obligation to protect classified information and abide by the law," said the spokesman, Shawn Turner.

The NSA has requested a criminal probe into the leaked information. On Sunday, the US Justice Department said it was in the initial stages of a criminal investigation following the leaks.

Booz Allen, a US management and technology consultancy, said reports of the leaked information were "shocking and if accurate, this action represents a grave violation" of company policy.

It said Snowden had been employed by the company for less than three months and that it would cooperate with any investigations.

A spokesman for Dell Inc declined to comment on reports that Snowden had been employed at that company. In 2009, Dell acquired Perot Systems, a US government contractor that did work for US intelligence agencies.

Snowden's decision to reveal his identity and whereabouts lifts the lid on one of the biggest security leaks in US history and escalates a story that has placed a bright light on Obama's extensive use of secret surveillance.

The exposure of the secret programs has triggered widespread debate within the United States and abroad about the vast reach of the NSA, which has expanded its surveillance dramatically in since the September 11 attacks on Washington and New York in 2001.

US officials say the agency operates within the law. Some members of Congress have indicated support for the NSA activities, while others pushed for tougher oversight and possible changes to the law authorizing the surveillance.