The United States has launched a criminal investigation and is taking "all necessary steps" to prosecute Edward Snowden for exposing secret US surveillance programmes, the director of the US Federal Bureau of Investigation (FBI) has said.

"As to the individual who has admitted to making these disclosures, he is the subject of an ongoing criminal investigation," FBI Director Robert Mueller told the House Judiciary Committee on Thursday.

"These disclosures have caused significant harm to our nation and to our safety. We are taking all necessary steps to hold the person responsible for these disclosures," he said.

The FBI chief's comments offered the first explicit confirmation that the US government was pursuing Snowden, the 29-year-old American IT specialist who has admitted to leaking information about far-reaching surveillance programmes.

Snowden, who worked as a sub-contractor handling computer networks for the National Security Agency (NSA), is now in Hong Kong, where he has vowed to contest any US attempt to extradite him.

He has said he plans to request asylum and that he divulged secrets to Britain's Guardian newspaper and the US-based Washington Post because he believed the US surveillance programmes were illegal and intrusive.

For the sake of national security

Mueller strongly defended the surveillance programmes, arguing that they had helped to protect US citizens and that leaking information on them harms US national security.

In his last appearance as FBI director before the committee, Mueller said that terrorists track leaked information "very, very closely" and that because of leaks "we lose our ability to get their communications" and "we are exceptionally vulnerable".

Responding to questions by committee chairman Bob Goodlatte, a Republican, Mueller said the Foreign Intelligence Surveillance Court has approved the surveillance programmes and they have been conducted in compliance with US law and with oversight from Congress.

The revelation that the NSA is collecting millions of US phone records along with digital communications stored by nine major Internet companies has touched off a national debate over whether the Obama administration, in its efforts to thwart terrorism, has overstepped proper bounds by using intrusive surveillance methods.

John Conyers, the committee's top Democrat, expressed concern that the two programmes were too far-reaching.

"It's my fear that we are on the verge of becoming a surveillance state," said Conyers.

On Wednesday, Goodlatte said that when it comes to national security leaks, it is important to balance the need to protect secrecy with the need to let the news media do their job.

The Justice Department revealed last month that it had secretly gathered phone records of The Associated Press and emails of Fox News journalist James Rosen in an effort to crack down on leakers of classified information.

"Over the past few years, we have witnessed troubling national security leaks and have learned that the Obama administration seems to be bending the rules in place that protect the freedom of the press in its investigations," Goodlatte said.