A B-52 bomber, mistakenly armed with six nuclear warheads, was flown for more than three hours across several U.S. states last week, prompting an air force investigation and the firing of one commander, Pentagon officials in Washington said Wednesday.

The officials said the plane was carrying advanced cruise missiles on Aug. 30 to Barksdale Air Force Base, La., from Minot Air Force Base, N.D, roughly 65 kilometres south of the Canadian border.

The officials spoke on condition of anonymity because of a U.S. Defence Department policy not to confirm information on nuclear weapons.

The Air Combat Command has ordered a command-wide stand-down on Sept. 14 to review procedures, the officials said. They said there was a minimal risk to crews and the public because of safety features designed into the munitions.

Representative Ike Skelton, a Democrat from Missouri and chairman of the House armed services committee, called the mishandling of the weapons "deeply disturbing" and said the committee would press the military for details.

"The American people, our friends, and our potential adversaries must be confident that the highest standards are in place when it comes to our nuclear arsenal," said Skelton in a statement.

The missiles, which are being decommissioned, were mounted on pylons on the bomber's wings. It is unclear why the warheads had not been removed before the flight.

In addition to the munitions squadron commander who was relieved of his duties, crews involved with the mistaken load — including ground crew workers — have been temporarily decertified for handling munitions, one official said.

The investigation is expected to take several weeks.

The incident was first reported in Military Times newspaper.