I think it's time for you to go, to be accountable to the American people

Committee chairman speaking to Energy Secretary Bill Richardson

But US Energy Secretary Bill Richardson told a Senate committee: "So far there is no evidence of espionage, nor is there evidence that the drives have ever left Los Alamos."

They were found earlier this month behind a copying machine at the New Mexico nuclear weapons research lab, despite being missed by an FBI search.

The drives contained secrets on US, Russian, Chinese and French nuclear systems to help a team of experts dismantle weapons in an emergency, such as a terrorist attack.

Richard Shelby, chairman of the Senate Intelligence Committee, reiterated an earlier call for Mr Richardson to resign.

'Unacceptable' delay

He said: "We need strong and consistent leadership at the top. It appears that we don't have it.

Bill Richardson: Hard drives did not leave Los Alamos

"I think it's time for you to go, to be accountable to the American people."

Although the hard drives were found to be missing on 7 May, the head of the lab, John Browne, was not told until 1 June.

"It is clear that there was a lengthy delay, internal to the laboratory, in notifying me," said Mr Browne. "That delay is plainly and absolutely unacceptable."

Mr Browne pledged disciplinary action, including sacking, against anyone "who wilfully or carelessly violated the rules".

Some critics of lax security at Los Alamos say the problem stems from a culture in which a desire for scientific freedom clashed with security needs.

Mr Browne said: "Arrogance, indifference, or carelessness, regardless of an individual's or an organisation's accomplishments, will not be allowed to compromise our nation's security."