The US air force has discharged 34 officers who control some of the world's deadliest weapons for cheating or tolerating cheating by others in proficiency tests.

The admission on Wednesday was yet another stunning setback for a nuclear missile force already beset by missteps and leadership lapses.

The cheating scandal is merely the latest in a series of air force nuclear stumbles in recent months, including deliberate violations of safety rules, failures of inspections, breakdowns in training and evidence that the officers who operate the missiles from underground command posts are suffering burnout.

In October, the nuclear missile force commander was fired for engaging in unseemly behaviour, including drunkenness, while leading a US delegation on a nuclear exercise in Russia.

"Profoundly disappointed" Air Force Secretary Deborah Lee James told journalists that cheating at Malmstrom Air Force Base, Montana, had been discovered during a probe of drug possession by 11 officers at several air force bases.

Chief of Staff General Mark Welsh admitted that it could be the biggest scandal in the history of the missile force.

A spokesman for Defence Secretary Chuck Hagel said the Pentagon chief, who just last week praised the force for its professionalism, was "deeply troubled" to learn of the cheating allegations.

The drug investigation that led to the discovery of alleged cheating was disclosed by the Pentagon last week.

It said that the cheating involved officers at six bases - five in the US and one in Britain.