The Pentagon is investigating how a B52 bomber was mistakenly armed with six nuclear warheads last week and allowed to fly 1,500 miles across America before anyone noticed the weapons were missing.

A squadron commander in charge of the warheads, each of which has up to ten times the destructive power of the Hiroshima atom bomb, has been relieved of his duties while crews responsible for the error have been banned from handling munitions.

Although officials insisted yesterday that there was never any danger of the warheads detonating, the disclosure will shake confidence in the security of America’s vast nuclear arsenal.

Washington has often voiced concerns about sloppy controls over nuclear material in the former Soviet Union, but this incident could not have been closer to home.

On August 30, a B52 took off from Minot Air Force Base, North Dakota, and flew for more than three hours before landing at Barksdale Air Force Base, Louisiana.

It was only then that crews discovered the six Advanced Cruise Missiles mounted on the bombers’ wings were armed with W80-1 warheads, which have yields of between 5 and 150 kilotons.

“Standards are very exacting when it comes to munitions handling,” said Air Force spokesman Lieutenant-Colonel Edward Thomas. “All evidence we have seen so far points to an isolated mistake.

“It is important to note that munitions were safe, secure and under military control at all times. The error was discovered by airmen during internal Air Force checks.”

An investigation headed by Major-General Douglas Raaberg, director of Air and Space Operations at Air Combat Command Headquarters, was started immediately to find the cause of the mistake and discover how strict safeguards apparently failed. The Air Combat Command has ordered a complete stand down on September 14 to review procedures, officials confirmed yesterday.

Military experts said that there was no risk of a nuclear detonation even if the B52 had crashed, because the warheads can only be activated with Presidential approval.

But Ike Skelton, chairman of the House Armed Services Committee, called the mishandling of the weapons “deeply disturbing”. “There is no more serious issue than the security of nuclear weapons,” said the Democratic congressman. “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.” Retired Major-General Donald Shepperd said: “This really needs to reviewed, investigated and fixed. This was serious.”