The two most senior officials in the US Air Force have been forced to resign following incidents that threw into question the safety of the country's nuclear weapons.

Robert Gates, the US Defence Secretary, accepted the resignations yesterday of Michael Wynne, the Air Force Secretary, and General Michael Moseley, the Chief of Staff, in the wake of two recent embarrassing incidents.

"I believe that we needed a change of leadership to bring a new perspective and to especially underscore the importance of accountability in dealing with these kinds of problems," said Mr Gates.

Last year the US Air Force mistakenly sent a shipment to Taiwan of four cone-shaped, electrical fuses used to trigger nuclear warheads for intercontinental missiles. The shipment was supposed to contain helicopter batteries. The mistake was only recently revealed.

In a further incident last year, a B-52 bomber was mistakenly armed with six nuclear-tipped cruise missiles and flown across the United States. The pilot and crew were not aware they had nuclear arms on board. That error was considered so grave that President Bush was quickly informed.

Mr Gates said the incidents had showed a lack of leadership and oversight within the top ranks of the Air Force.

Others pointed to last month's discovery that Air Force 5th Bomb Wing had failed a defence "nuclear surety" inspection despite having months to prepare and receiving heightened scrutiny following the previous incidents. The inspection found deficiencies in the wing's ability to protect its part of the nation's nuclear stockpile.

"The stewardship of our nuclear deterrent is the most sensitive mission that we have," said Mr Gates. "Therefore, I think the problems that have been identified, despite the fact there was no compromise of the technology, despite the fact there was no danger involved, the fact that the stewardship itself and the declining standards raised questions in the minds of the public as well as internationally, in my view, required strong action."

The resignations followed the publication of a report on a Navy admiral's investigation that criticised the Air Force's reaction to the incidents. Mr Gates said the report "depicts a pattern of poor performance" in which Air Force staff failed to improve security following mishaps and Air Force personnel handling nuclear weapons consistently failed to follow existing rules.

Air Force leaders "not only fell short in terms of specific acts, they failed to recognize systemic problems, to address those problems or, when beyond their authority to act, to call the attention of superiors to those problems," said Mr Gates.

Mr Gates described the Taiwan incident as "the trigger" for the resignations but said the investigation had also uncovered a "lack of effective air force leadership oversight" as well as the "lack of a critical self-assessment culture". Mr Gates said it was the report on the Taiwan incident, written by Admiral Kirkland Donald, that had led him to request the resignations.

Mr Gates has asked former defence secretary James Schlesinger to lead a task force to develop recommendations to ensure improved levels of accountability and control are maintained for nuclear weapons, delivery vehicles and sensitive components.

"Our policy is clear," said Mr Gates. "We will ensure the complete physical control of nuclear weapons and we will properly handle their associated components at all times. It is a tremendous responsibility and one we must and will never take lightly."

Senator Carl Levin, the Democratic chairman of the Senate Armed Service committee, welcomed the resignations.

"Secretary Gates's focus on accountability is essential and had been absent from the office of the Secretary of Defense for too long," said Mr Levin. "The safety and security of America's nuclear weapons must receive the highest priority, just as it must in other countries."

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