The lapse was reported to President George W. Bush after the nuclear warheads were discovered when the aircraft landed at Barksdale Air Force Base in Louisiana, a military official said on condition of anonymity.

An air force official, who also asked to remain unnamed, said the B-52, which originated at Minot Air Base in North Dakota, had six cruise missiles with nuclear warheads loaded on pylons under its wings.

The US Air Force has relieved the munition squadron commander at Minot Air Base in North Dakota of his duties, and launched an intensive investigation into the August 30 incident, a Pentagon spokesman said.

"At no time was there a threat to public safety," said Lieutenant Colonel Ed Thomas.

"It is important to note that munitions were safe, secure and under military control at all times."

The Pentagon would not provide specifics, citing secrecy rules, but an expert said the incident was unprecedented, and pointed to a disturbing lapse in the air force's command and control system.

"It seems so fantastic that so many points, checks can dysfunction," he said Hans Kristensen, an expert on US nuclear forces.

"We have so many points and checks specifically so we don't have these kinds of incidents," he said.

The breach originally was reported by the Military Times newspaper Wednesday and was confirmed by the Pentagon later in the day.

US Defense Secretary Robert Gates was notified early Friday of the incident by Air Force chief of staff General Michael "Buzz" Moseley, Pentagon press secretary Geoff Morrell said.

"I can also tell you that it was important enough that President Bush was notified of it," Morrell said.

Gates has been getting daily briefings from Moseley on the incident, and expects a report by the end of next week, he said.

"The munitions squadron commander has been relieved of his duties, and final action is pending the outcome of the investigation," he said.

"In addition, other airmen were decertified from their duties involving munitions."

Kristensen said he knew of no other publicly acknowledged case of live nuclear weapons being flown on bombers since the late 1960s.

The nuclear weapons expert said the air force keeps a computerized command and control system that traces any movement of a nuclear weapon so that they have a complete picture of where they are at any given time.

He said there would be checks and detailed procedures at various points from the time they are moved out of bunkers until they are loaded onto planes, and flown away.

"That's perhaps what is most worrisome about this particular incident -- that apparently an individual who had command authority about moving these weapons around decided to do so," he said.

"It's a command and control issue and it's one that calls into question the system, because if one individual can do that who knows what can happen," he said.

Nuclear weapons are normally transferred on cargo planes, never on the wings of bombers, Kristensen said. Bomber flights with live nuclear weapons were ended in the late 1960s after accidents in Spain in 1966 and in Greenland in 1968.

The weapons were among 400 advanced cruise missiles that the Defense Department quietly decided to retire in March over the course of this year.

The advanced cruise missile is a stealthy, longer range version of the air launched cruise missile first deployed in the early 1980s.

They carry W-80 warheads of up to 150 kilotons, ten times the destructive force of the bomb dropped on Hiroshima.

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