His comments came at the end of a two-day swing through US air bases in which he pressed the air force to give priority attention to its nuclear forces, warning that the need for deterrence was growing, not diminishing.

"Every senior leader, when you're asked what keeps you awake at night, it's the thought of a terrorist ending up with a weapon of mass destruction, especially nuclear," Gates told reporters on a flight back to Washington.

Gates acknowledged that deterrence would likely have little effect on terrorists prepared to die for their cause.

But, he said, "It certainly plays a part in deterring the states who might be a source for it (nuclear material.)"

He said it was hard to imagine that terrorists could obtain nuclear materials from anywhere other than a state.

Asked what the consequences would be for a state that supplied nuclear weapons used in a terrorist attack on the United States, Gates said, "The way we've always framed it is the consequences for a state who unleashed a weapon of mass destruction on the United States would be catastrophic.

"We went through this in the lead up to the first Gulf War. We never were explicit about it. We just said the consequences would be catastrophic. It is best to leave it ambiguous."

The subject came up earlier in a closed question and answer session Gates had with air force personnel at Peterson Air Force Base in Colorado, Pentagon press secretary Geoff Morrell said.

In discussing the growing importance of deterrence after a post-Cold War lull, Gates also cited Russia's recent investments in its nuclear arsenals and challenges posed by China and Iran, he said.

The United States is convinced that an Iranian enrichment uranium program is part of a drive to acquire nuclear weapons, despite Iranian denials.

Gates's visit to air bases also follows recent revelations of mishandling of US nuclear material.

Last week, the defense chief fired the air force's two top leaders after an investigation found a 10-year slide in standards, performance and oversight over the country's nuclear forces.

The investigation was prompted by the discovery of a mis-shipment of fuses for nuclear weapons to Taiwan that went unnoticed for 18 months.

Six months earlier a B-52 bomber flew across the United States, inadvertently carrying nuclear armed cruise missiles under its wing.

In speeches to personnel at air bases in Virginia, Colorado and Illinois, Gates said he had no choice but to fire the air force chief of staff and the air force secretary because of the systemic nature of the failings and the importance of the air force's nuclear mission.

"That stewardship is our most sensitive mission," he told personnel at Scott Air Force Base in Illinois, home of the Air Mobility Command in a speech.

Gates also met briefly with General Norton Schwartz, who has been nominated to become the next air force chief of staff, officials said.

Schwartz currently heads the US Transportation Command headquartered at Scott Air Force Base, managing the movement of troops and equipment around the world.

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