The US military has suspended 34 officers in charge of launching nuclear missiles for cheating on a proficiency test, Air Force leaders said Wednesday.

The scandal at Malmstrom Air Force Base in Montana marked the latest in a series of damaging revelations dogging the country's nuclear force, including a separate probe into illegal drugs that came to light last week.

"There was cheating that took place with respect to this particular test," Air Force Secretary Deborah Lee James told a news conference.

"Some officers did it. Others apparently knew about it, and it appears that they did nothing, or at least not enough, to stop it or to report it."

She called the cheating "absolutely unacceptable behavior" but insisted that the nuclear weapons had been safe all along.

"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," said James.

Officials said it was the largest scale cheating scandal ever in the nuclear force, as it implicated roughly 20 percent of the Malmstrom base's 190-member launch officer corps.

The Air Force also stripped the officers of their security clearances over the allegations.

The cheating came to light as criminal investigators were looking into alleged illicit drug possession by some officers at Malmstrom and other bases.

Two of the officers implicated in the cheating also are linked to the separate drug case, which involves 11 airmen at six bases, officials said.

The Air Force has come under growing scrutiny over a spate of setbacks linked to the nuclear force, amid persistent reports of low morale among the troops assigned to the mission.

The work is seen by many airmen as a less promising career involving monotonous shifts.

In a recent embarrassing incident, the head of the 20th Air Force, Major General Michael Carey, who oversaw the country's land-based intercontinental ballistic missiles, was sacked in October after he went on a drunken bender in a work trip to Russia.

The two-star general was drunk through much of his official four-day visit to Russia, insulting his hosts and cavorting with "suspect" women, according to an inspector general's report.

As a result of the cheating case, all launch officers in the missile force have been ordered to take the proficiency exam again and the testing will be completed by Thursday, said James.

"People at every level will be held accountable if and where appropriate," said the Air Force chief of staff, General Mark Welsh.

The four-star general said he and James planned to visit all missile bases in the coming days and deliver a stern message that "there is nothing more important to the nation than the integrity and the trustworthiness of the people who defend it and that anyone who doesn't understand that should find another line of work."

Defense Secretary Chuck Hagel "was deeply troubled to learn of these allegations," his spokesman Rear Admiral John Kirby said in a statement.

Hagel, who visited a missile base in Wyoming last week, "strongly supports the aggressive steps the Air Force is taking in response" and will be following the issue closely, Kirby said.

But the Air Force chief said the nuclear mission was not in jeopardy and that cheating on one exam did not cast doubt on the integrity of the whole organization.

He said that "there are multiple tests and outside inspections and all these checks and balances."

He also said new leaders had just been put in place in the missile force.

Newly appointed officers had recently taken over at the 20th Air Force, three missile wings under that command and the Air Force's Global Strike Command, Welsh said.