The US Air Force said Thursday that 92 nuclear missile officers are now implicated in a widening scandal over cheating on exams as officials cited "systemic problems" in the force.

In the latest setback for the troubled nuclear mission, Air Force Secretary Deborah Lee James said she returned from a visit to missile bases believing the cheating was part of a broader morale problem among launch officers.

"As the investigation has moved forward, we can now report there is a total of 92 crew members that have been identified as having some level of involvement," James told a news conference.

"That means either participating in the cheating or knowing something about it and not standing up and reporting it," she said.

Two weeks ago, officials said 34 officers were implicated at Malmstrom Air Force base in Montana.

The latest tally of 92 launch officers ensnared by the scandal represents about half of the total 190-member officer corps of the Montana base, and nearly 20 percent of the roughly 500 officers who run the missile force.

The mounting scandal, as well as other embarrassing incidents, have prompted commanders to put a hold on any promotions of senior officers in the nuclear mission, a defense official, who spoke on condition of anonymity, told AFP.

"They're reviewing all of those (proposed promotions)," the official said.

While acknowledging serious questions about the working climate and leadership of the nuclear force, James reiterated the Pentagon's stance that the destructive weapons were in safe, competent hands.

"I remain confident -- and having gone there to our bases last week, even more confident -- in the safety, reliability, and effectiveness of the nuclear mission," she said.

The cheating was discovered when criminal investigators looking into illegal drug possession among missile launch officers stumbled upon evidence of cheating on a monthly proficiency test at the Montana base.

The investigators found one officer had the answers to a monthly proficiency test on his mobile phone and had sent them to his fellow crew members.

James said the illicit drug case has also widened and now two more officers have been implicated, bringing the total to 13 airmen tied to the probe.

Since the end of the Cold War, which reduced the perceived relevance of the force, the Pentagon has worried about the morale and professionalism of the officers who oversee the land-based intercontinental ballistic missiles (ICBMs).

With 450 ICBMs at three bases in Montana, North Dakota and Wyoming, more than 500 officers manage the weapons around the clock in steel cocoons 100 feet (30 meters) underground, rehearsing launch protocols in often tedious routines.

James said she held confidential discussions with small "focus groups" of airmen at the missile bases this month and came away persuaded that changes were needed in how personnel are managed and promoted.

She said frequent exams are the overriding criteria that determine promotion, and as a result launch officers are fixated with scoring perfect grades as anything less than 100 percent can derail a career.

"I heard repeatedly from teammates that the need for perfection has created a climate of undue stress and fear," she said.

"Fear about the future. Fear about promotions. Fear about what will happen to them in their careers."

She said members of the nuclear missile force have doubts about their career prospects and some question if the Pentagon genuinely values their mission.

Since the cheating problems were exposed, Defense Secretary Chuck Hagel has ordered a sweeping review of the nuclear force.

On Wednesday, Hagel convened an extraordinary meeting of nuclear commanders at the Pentagon to discuss the state of the mission and questions about morale.

Hagel has said he is open to suggestions to raise the pay of the ICBM officers, an idea floated in the past.

In 2008, concerns about the professionalism of the nuclear force prompted the then defense secretary Robert Gates to sack the Air Force chief and the civilian secretary after incidents in which atomic weapons were mishandled.

Afterward, a special Pentagon review of nuclear forces concluded the mission had been neglected and marginalized since the end of the Cold War.