WASHINGTON — An Air Force B-52 bomber carrying six cruise missiles armed with nuclear warheads flew a cross the central United States last week after the nuclear weapons were mistakenly attached to the airplane’s wing, defense officials said Wednesday.

The Stratofortress bomber, based at Minot Air Force Base in North Dakota, was flying a dozen Advanced Cruise Missiles to Barksdale Air Force Base in Louisiana on Aug. 30. But crews inadvertently loaded half of them with nuclear warheads attached.

Air Force officials said the warheads were not activated for use and at no time posed a threat to the public. Military officials were more concerned that the warheads were unaccounted for during the several hours that the missiles were in transit. The missiles never left Air Force control, they said.

The aircraft’s pilots and crew were unaware that they were carrying nuclear warheads, officials said. Airmen in Louisiana discovered the error after the bomber arrived.

The incident, first reported by the Military Times newspapers, prompted senior leaders to relieve a munitions squadron commander of his duties.

The missiles, part of an Air Force fleet of more than 450 of their kind, are being phased out and normally would not carry nuclear warheads while in transit. Defense officials said the B-52’s mission last Thursday did not include training runs, so the missiles would not have been launched.