Replacements for the two men have not yet been announced
The US air force's two most senior officials have resigned after a report suggested it was performing poorly in its handling of nuclear arms and parts.
Gen T Michael Moseley, the chief of staff, and Air Force Secretary Michael Wynne were both asked to resign by Defence Secretary Robert Gates.
Mr Gates told reporters that the security of US nuclear weapons and parts had been in question.
One incident involved the sending of nuclear weapons fuses to Taiwan.
Mr Gates said he had accepted the two men's resignations and that he would announce their replacements at a later time.
He also announced that former Defence Secretary James Schlesinger would head a senior level task force to "recommend improvements necessary to ensure that the highest levels of accountability and control are maintained in the stewardship of nuclear weapons".
Cruise missile mix-up
Mr Gates cited two embarrassing incidents. In the first, electrical fuses for ballistic missiles were mistakenly sent to Taiwan in 2006 in place of helicopter batteries.
The fuses, designed for the nose cone of a nuclear missile, were sent from a US airbase in Wyoming.
The B-52 crew had been unaware they were carrying nuclear missiles
The Chinese government, which vehemently opposes US arms sales to Taiwan and has threatened to attack the island if it declares independence, was informed by the US about the error.
The other embarrassing incident, Mr Gates added, was a flight across the US by a B-52 bomber mistakenly armed with six nuclear-tipped cruise missiles, without anyone realising the weapons were on board.
Three colonels, a lieutenant-colonel and 66 other personnel were punished following the incident at Minot Air Force Base, North Dakota, on 29 August last year.
Speaking in Washington, Mr Gates said his decision to request the two resignations had been based on the findings of an investigation into the Taiwan error by Adm Kirkland Donald.
The admiral had found a "lack of a critical self-assessment culture" in the air force.
"The Taiwan incident clearly was the trigger," Mr Gates said but the Donald investigation also found a "lack of effective air force leadership oversight", he added.
The BBC's Kevin Conolly in Washington says the defence secretary now faces the huge task of restoring faith in the whole issue of how the American armed forces handle their huge nuclear arsenals
These sackings are the first step, our correspondent says.
In his resignation statement, Gen Moseley said he took "full responsibility for events which have hurt the Air Force's reputation or raised a question of every airman's commitment to our core values".