The U.S. Air Force confirms that nuclear warheads were mistakenly flown over America in August, calling the error "unprecedented".

Air Force Secretary Michael Wynne said Friday several commanders and dozens of servicemen involved in the incident have been relieved of their duties.

Wynne described a series of errors that allowed a package of six nuclear armed missiles to be loaded mistakenly onto a B-52 bomber at Minot Air Force Base in North Dakota. The weapons should have been been disarmed before their transfer to Barksdale Air Force Base in Louisiana, where they were to be destroyed.

The Air Force secretary called the incident isolated. He attributed it to a lack of attention to detail.

Defense experts have said there was no risk of nuclear detonation even if the plane had crashed, because of safety features built into the warheads.

Wayne Friday added that despite the errors, the warheads were always under the protection of Air Force security.

He also said the incident continues to be under review. He did not rule out criminal proceedings against those involved.

The incident has been described as the biggest breach of nuclear weapons security in decades.

A Pentagon spokesman said at the time that the incident was so serious that President Bush and Defense Secretary Robert Gates were told of it right away.