US Defense Secretary Robert Gates ousted the Air Force's top military and civilian leaders Thursday, holding them to account in a historic Pentagon shake-up after embarrassing nuclear mix-ups. 

Gates announced at a news conference that he had accepted the resignations of Air Force Chief of Staff Gen. Michael Moseley and Air Force Secretary Michael Wynne – a highly unusual double firing.

Gates said his decision was based mainly on the damning conclusions of an internal report on the mistaken shipment to Taiwan of four Air Force electrical fuses for ballistic missile warheads. And he linked the underlying causes of that slip-up to another startling incident: the flight last August of a B-52 bomber that was mistakenly armed with six nuclear-tipped cruise missiles.

The report drew the stunning conclusion that the Air Force's nuclear standards have been in a long decline, a "problem that has been identified but not effectively addressed for over a decade."

Gates said an internal investigation found a common theme in the incidents, "a decline in the Air Force's nuclear mission focus and performance" and a failure by Air Force leaders to respond effectively.

In a reflection of his concern about the state of nuclear security, Gates said he had asked a former defense secretary, James Schlesinger, to lead a task force that will recommend ways to ensure that the highest levels of accountability and control are maintained in Air Force handling of nuclear weapons.

In somber tones, Gates told reporters his decision to remove Wynne and Moseley was based on the findings of an investigation of the debacle by Adm. Kirkland Donald. The admiral found a "lack of a critical self-assessment culture" in the Air Force nuclear program, making it unlikely that weaknesses in the way critical materials such as nuclear weapons are handled could be corrected, Gates said.

Gates said Donald concluded that many of the problems that led to the incidents "have been known or should have been known."

The Donald report is classified; Gates provided an oral summary.