WASHINGTON (CNN) -- The top military and civilian leaders of the U.S. Air Force were forced out Thursday over the handling of nuclear weapons, the Defense Department secretary said.

Air Force Chief of Staff Gen. Michael Moseley, left, and Secretary Michael Wynne have stepped down.

Chief of Staff Gen. T. Michael Moseley and Secretary Michael W. Wynne resigned over the department's concern over two incidents, including the August flight of a B-52 bomber that flew across the country with nuclear weapons.

"Focus of the Air Force leadership has drifted" in terms of handling nuclear weapons and equipment, Defense Secretary Robert Gates said.

In August, a B-52 bomber flew from North Dakota to Louisiana with the crew unaware that six nuclear-tipped missiles were on board. Four officers were relieved of duty afterward, including three colonels.

Gates also cited this year's discovery that components designed to arm and fuse nuclear warheads were accidentally shipped to Taiwan in 2006. Watch Gates talk about the firings »

Critics also cite last month's news that the Air Force's 5th Bomb Wing failed a defense "nuclear surety" inspection -- despite having months to prepare and being under close scrutiny after the previous incidents. The inspection found deficiencies in the wing's ability to protect its part of the nation's nuclear stockpile.

The resignations come after a report on a Navy admiral's investigation that criticized the Air Force's reactions to the incidents.

Gates said the report "depicts a pattern of poor performance" in which Air Force brass didn't act to improve security after 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," Gates said.

The investigation found that although the Taiwan incident didn't compromise the integrity of the U.S. nuclear force, it represented "a significant failure" by the Air Force to ensure sensitive military components, Gates said.

Gates said the report concluded that erosion in the branch's command and oversight standards helped lead to the incidents, and that they could have been prevented if the oversight programs had functioned correctly.

He said he asked for the resignations after consulting President Bush.

Sen. Carl Levin, the Democratic chairman of the Senate Armed Service committee, welcomed Gates' decision.

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

Rep. Edward J. Markey, a Massachusetts Democrat and a senior member of the House Homeland Security Committee, also welcomed the resignations.

Markey said he's long been concerned about what he called a series of dangerous Air Force missteps in handling nuclear weapons.

"The magnitude and frequency of these errors indicate a deep-seated problem within Air Force culture, practice and training," Markey said. "The entire Department of Defense should immediately recommit itself to ensuring the safety and security of our nuclear stockpile before one of these mistakes has lethal consequences."

A senior military source said the August nuclear weapons incident was the straw that broke the camel's back but that other leadership issues also factored into Moseley's and Wynne's resignations.

A previous investigation into the B-52 flight uncovered a "lackadaisical" attention to detail in day-to-day operations at the air bases involved, an Air Force official said in October.

Maj. Gen. Dick Newton, assistant deputy chief of staff for operations, said the investigation found "a failure to follow procedures" by "a limited number of airmen" at the two bases. Newton defended the procedures themselves.

Wynne is not the first secretary of a military branch to resign since Gates became Defense secretary.

In March 2007, Gates announced the resignation of Army Secretary Francis J. Harvey in the wake of reports of substandard conditions at Walter Reed Army Medical Center in Washington.

CNN's Jamie McIntyre and Barbara Starr contributed to this report.

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