US Air Force officers in charge of launching nuclear missiles have been punished twice this year for leaving open a blast door at their command posts, officials said Tuesday.

The two-man crews, who control the launch keys for nuclear missiles, are forbidden from opening such doors at their underground "capsules" unless both officers are awake to ensure strict control of the destructive weapons.

"There were two cases in the last year where non-judicial punishment was given for a violation for leaving the blast doors open when they should have been shut," said Lieutenant Colonel John Sheets, spokesman for the military's Global Strike Command.

At Minot Air Force Base in North Dakota in April, while one crew member was napping, his counterpart opened the blast door when a cook came to deliver their meals, Sheets said.

And at Malmstrom Air Force base in Montana in May, one officer left the door open when a maintenance team came do repairs, even though his fellow crew member was asleep in his quarters, the spokesman said.

In both cases, the napping officer should have been wakened before the blast door was open, Sheets said.

The incidents were first reported by the Associated Press.

Drafted during the Cold War era, the rules for the blast door are designed to ensure strict control and security for the intercontinental ballistic missiles.

The incidents are the latest in a string of embarrassing episodes for the US military's nuclear mission. Inspections at missile bases this year exposed problems and two top officers who oversee the arsenal were sacked from their posts this month over alleged personal misconduct.

Some skeptics say the nuclear forces have become irrelevant with the demise of the Soviet Union and that Air Force officers do not see the mission as a promising career path.

But the Air Force insisted the incidents do not reflect a deeper problem with the safety and security of the nuclear arsenal.

"Our command is still focused on excellence," Sheets told AFP.

"They strive for perfection. And when there are issues, when people fall short, they are held accountable," he said.

The two-man crews at both sites received administrative punishment from commanders at the bases over the incidents.

At Minot, the crew commander had to forfeit wages over a two-month period while the other crew member was given a letter of "admonishment," according to Sheets.

At Malmstrom, meanwhile, the crew commander had to forfeit wages for a two-month period and is facing a possible discharge. The other crew member was given a letter of reprimand.