The "missileers" who oversee America's land-based nuclear arsenal were once seen as the tip of the spear for the US military during the tense days of the Cold War.

But now the crews face questions about their discipline, their professionalism and even the rationale for their job.

Revelations this week that missile launch officers cheated on a proficiency exam -- the latest in a stream of embarrassing incidents -- has put the spotlight on a force that has posed a recurring headache to commanders for years.

After announcing 34 officers had been suspended over the cheating at Malmstrom base in Montana, Air Force leaders called the scandal "unacceptable" and vowed to rectify the problem.

But concerns about declining standards in the intercontinental ballistic missile (ICBM) force are nothing new.

The trouble began after the demise of the Soviet Union, as the mission gradually received a lower priority and offered a less promising career path.

"Since the end of the Cold War, the Air Force level and intensity of concentration on its nuclear mission has declined conspicuously," a Pentagon review found in 2008.

The report was ordered by former defense secretary Robert Gates who fired the Air Force chief and civilian secretary after accounts of negligent handling of nuclear weapons.

At the time, Gates said there were signs of "a degradation of the authority, standards of excellence, and technical competence within the nation's ICBM force."

Reforms were enacted, but inspections over the past year have turned up fresh problems at the missile sites.

In recent months, two senior commanders have been sacked for misbehavior, including the head of the ICBM force after he went on a drunken bender in a trip to Russia.

In October, officials said missile officers were caught twice failing to close the blast doors on their bunkers, violating a strict security rule. And authorities say crew members are under criminal investigation for illegal drug possession.

Air Force officers say the incidents do not add up to a crisis. But critics contend crews on nuclear-armed submarines or bombers do not seem to suffer the same level of disciplinary or performance problems.

'Pride in the mission'

Over the years, the Air Force has tried to boost the esprit de corps of the crews by introducing special uniforms and badges. And proposals to boost pay for the missile crews have been discussed for more than a decade.

While expressing shock at the cheating revelations, General Mark Welsh, the Air Force chief of staff, insisted on Wednesday the nuclear missiles were in safe, extremely competent hands.

The four-star general, however, acknowledged morale is not what it was during the Cold War, when working in Strategic Air Command (SAC) carried prestige and a path to promotion.

In that era "there was a pride in the mission," Welsh said. "There was a feeling that the mission was critically important."

He said the Air Force had to ensure crews understood their jobs remained just as vital, maintaining missiles that officials say serve as a "deterrent" against America's adversaries.

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 again and again.

A former launch officer, John Noonan, has described the work as often tedious and solitary, except for the occasional "virtual" nuclear war exercise.

"Being a missileer means that your worst enemy is boredom. No battlefield heroism, no medals to be won," Noonan wrote in Wired magazine's "Danger Room" blog in 2011.

"The duty is seen today as a dull anachronism."

Other missile veterans still see the work as necessary and argue there is no collapse in morale among the crews.

For arms control advocates, the whole enterprise is absurd and the discipline problems are an inevitable byproduct.

"There's no purpose to their mission anymore," said Joseph Cirincione, president of the Ploughshares Fund, which promotes reducing nuclear weapons stockpiles.

The launch officers "pull 12, 24-hour shifts in underground bunkers waiting to push a button they know they're never going to push," said Cirincione, an author who also has served as an adviser to President Barack Obama's administration.

Commanders have said they would support reducing the costly arsenal of land-based nuclear missiles, as weapons aboard submarines are able to reach any target on the globe. But lawmakers whose states host the silos oppose any cutbacks, he said.

"This is an outdated command, fielding obsolete weapons...," he said.

"The only mission for the ICBM is to immolate millions of innocent civilians."