WASHINGTON AFP) - The United States has launched a criminal investigation and is taking "all necessary steps" to prosecute Mr Edward Snowden for exposing secret US surveillance programs, the FBI director said on Thursday.

"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.

"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 Mr Snowden, the 29-year-old American IT specialist who has admitted to leaking information about far-reaching surveillance programmes.

Mr Snowden, who worked as a subcontractor handling computer networks for the National Security Agency, is now in Hong Kong, where he has vowed to contest any US attempt to extradite him.

Mr Mueller defended the collection of American phone records and Internet traffic as legal programmes that were approved by a judge and were in accordance with the Constitution.

He added that the government was determined to safeguard privacy rights and civil liberties even as it seeks to prevent possible terrorist attacks on the United States.