A US air force investigation into illegal drug use by officers charged with overseeing and launching nuclear missiles expanded on Wednesday when the military announced the suspension of dozens of additional officers for cheating on proficiency exams.

The cheating came to light during the investigation of the drug scandal, the air force said. The drug probe was first announced last Thursday.

In all, 11 air force officers are suspected of illegal drug use, and 34 officers have been implicated in cheating, according to the military.

"This is not about the compromise of nuclear weapons," air force chief of staff Mark Welsh said in a statement broadcast on the Pentagon Channel. "It's about the compromise of the integrity of some of our airmen … Our actions as we move forward will be about making sure that every member of our air force understands that we will not accept or allow that type of behaviour."

Welsh said it could be the biggest such scandal in the history of the intercontinental ballistic missile (ICBM) force.

The air force secretary Deborah Lee James, the service's top civilian official, told reporters at the Pentagon that she was "profoundly disappointed" about the alleged cheating at Malmstrom air force base, Montana. "This is absolutely unacceptable behaviour," James said of the cheating.

A spokesman for the secretary of defense, Chuck Hagel said he was "deeply troubled" to learn of the cheating allegations.

James said she will travel to each of the air force's three nuclear missile bases next week on a fact-finding mission to learn more about conditions within the missile launch force and the more senior officers who manage them. She suggested that the cheating was confined to this single case involving 34 officers, although numerous missile officers have told the Associated Press in confidence that some feel compelled to cut corners on their monthly proficiency tests because of intense pressure to score at the highest levels to advance in the force.

"I want all of you to know that, based on everything I know today, I have great confidence in the security and the effectiveness of our ICBM force," she said. "And, very importantly, I want you to know that this was a failure of some of our airmen. It was not a failure of the nuclear mission."

James, who has been in the job only four weeks, said the entire ICBM launch officer force of about 600 is being retested this week.

The drug investigation that led to the discovery of alleged cheating was disclosed by the Pentagon last week. It said then that it involved 10 officers at six bases — five in the US and one in the UK. On Wednesday, the air force said the number of suspects has grown to 11. Welsh said he could not comment further on the drug probe.

Missile-launch officers work in part in underground bunkers on bases equipped with nuclear-tipped intercontinental ballistic missiles.

Revelations of misconduct and incompetence in the nuclear missile program go back at least to 2007, when six nuclear-tipped cruise missiles were accidentally loaded onto a B-52 bomber in Minot, North Dakota, and flown to a base in Louisiana.

Last March, military inspectors gave officers at the ICBM base in Minot the equivalent of a "D" grade for launch mastery. A month later, 17 officers were stripped of their authority to launch the missiles.

In October, a senior air force officer in charge of 450 ICBMS, major general Michael Carey, was fired after accusations of drunken misconduct during a summer trip to Moscow. An internal investigation found that Carey drank heavily, cavorted with two foreign women and visited a nightclub called La Cantina, where "Maj Gen Carey had alcohol and kept trying to get the band to let him play with them."

The Associated Press contributed to this report