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Air Force Deputy Chief of Staff Maj. Gen. Richard Newton III acknowledged that flying nuclear-armed missiles from an air base in North Dakota to a base in Louisiana in late August was an "unacceptable mistake." ((Heesoon Yim/ Associated Press))

In its first explicit confirmation that six nuclear-armed missiles were erroneously flown from North Dakota to Louisiana in late August, the United States Air Force called the episode an "unacceptable mistake" on Friday.

It was a mistake that had never happened before.

"We are making all appropriate changes to ensure this has a minimal chance of ever happening again," Air Force Secretary Michael Wynne told reporters.

He spoke at a Pentagon news conference after Defence Secretary Robert Gates was briefed on the results of the air force's investigation into the Aug. 29-30 incident — one of the worst known breaches of nuclear weapons handling procedures in decades.

Appearing with Wynne was Maj.-Gen. Richard Newton, the air force deputy chief of staff for operations, who attributed the episode to an "unprecedented string of procedural errors."

The mistakes began with a failure by airmen to conduct a required inspection of the missiles before they were loaded aboard a B-52 bomber that flew from Minot Air Force Base, N.D., to Barksdale Air Force Base, La.

"This was a failure to follow procedures — procedures that have proven to be sound," Newton said.

A six-week investigation found fault with several officers, who have been relieved of duty, Newton said. The 5th Bomb Group commander at Minot was relieved of command, among others. Newton did not name them.

Newton said the 5th Bomb Wing, which operates the B-52 has been "decertified from its wartime mission."

He added that the August incident was isolated but a result of a problem at those two air bases.

"There has been an erosion of adherence to weapons handling standards at Minot Air Force Base and Barksdale Air Force Base," Newton said.

After arriving at Barksdale, the B-52 sat on a runway for hours with the missiles before the breach wasrealized — meaning a total of 36 hours passed before the missiles were properly secured, officials said.

The Air Combat Command ordered a command-wide stand-down — instituted base by base and completed Sept. 14 — to set aside time for personnel to review procedures, officials said.

The incident was so serious that it required President George W. Bush and Gates to be quickly informed.

Wynne prefaced his remarks about the B-52 incident by saying that in publicly confirming that nuclear weapons were involved he had authorized a one-time exception to U.S. policy, which states that the location of nuclear weapons will never be confirmed publicly. He said he made the exception because of the seriousness of the episode and its importance to the nation.

The weapons involved were the Advanced Cruise Missile, a "stealth" weapon developed in the 1980s with the ability to evade detection by Soviet radar. The Air Force said in March that it had decided to retire the Advanced Cruise Missile fleet soon, and officials said after the breach that the missiles were being flown to Barksdale for decommissioning but were supposed to be disarmed.