An air force unit that operates one-third of the nation's land-based nuclear missiles has failed a safety and security inspection, marking the second major setback this year for a force charged with the military's most sensitive mission, the general in charge of the nuclear air force told the Associated Press on Tuesday.

Lt Gen James M Kowalski, commander of Air Force Global Strike Command, said a team of "relatively low ranking" airmen failed one exercise as part of a broader inspection, which began last week and ended Tuesday. He said that for security reasons he could not be specific about the team or the exercise.

"This unit fumbled on this exercise," Kowalski said by telephone from his headquarters at Barksdale air force base, Louisiana, adding that this did not call into question the safety or control of nuclear weapons at Malmstrom air force base in Montana.

"The team did not demonstrate the right procedures," he said, and as a result was rated a failure.

To elaborate "could reveal a potential vulnerability" in the force, Kowalski said.

In a written statement on its website, Kowalski's command said there had been "tactical-level errors" in the snap exercise, revealing "discrepancies."

Without more details it is difficult to reliably judge the extent and severity of the problem uncovered at Malmstrom, home of the 341st Missile Wing, which is one of three nuclear missile wings. Each wing operates 150 Minuteman 3 intercontinental ballistic missiles, or ICBMs, on alert for potential launch against targets around the globe.

On Capitol Hill, a spokesman for Howard "Buck" McKeon, a California Republican and chairman of the House armed services committee, said that "two troubling inspections in a row at two different missile wings is unacceptable" to McKeon.

"It is his sense that the air force must refocus on the nuclear mission," spokesman John Noonan said. "The air force should hold failed leadership at the group and wing level accountable, recommit itself from the top down to the nuclear deterrent mission, and ensure a daily focus on its centrality to our nation's security."

Asked whether the air force intends to take disciplinary action against anyone for the inspection failure, Kowalski said the air force is "looking into it."

Overall, the 341st wing "did well," he said, earning ratings of excellent or outstanding in the majority of the 13 areas in which it was graded by inspectors. Those areas include management, administration, safety, security, emergency exercises, worker reliability and other facets of a mission that relies on teams of officers and enlisted personnel.

ICBM wings undergo two types of inspections. The one at Malmstrom was a "surety" inspection, which the Pentagon defines as "nuclear weapon system safety, security and control." The point is to ensure that no nuclear weapon is accidentally, inadvertently or deliberately armed or launched without presidential authority.

Kowalski said his command's inspector general has conducted 14 such inspections since early 2010 with just two failures — both involving the 341st wing. The first was in February 2010. The second was this week.

The 341st also failed a safety and security inspection in 2008.

A different type of inspection of the 91st Missile Wing at Minot air force base, North Dakota, in March of this year led the deputy commander of the wing's operations group to complain of "rot" in the force. Technically, the wing passed that inspection but its missile crews earned the equivalent of a "D" grade when tested on their mastery of Minuteman 3 launch operations using a simulator. The following month the 91st temporarily removed 17 officers from launch control duty — the first time such a large number had been pulled from duty.

In June, the commander in charge of training and proficiency of missile crews at Minot, Lt Col Randy Olson, was relieved of duty, citing a "loss of confidence" in his leadership.

Launch operations were not part of the Malmstrom inspection failure, Kowalski said.

The trouble at Minot was the latest in a longer series of setbacks for the air force's nuclear mission, highlighted by a 2008 Pentagon advisory group report that found a "dramatic and unacceptable decline" in the air force's commitment to the mission, which has its origins in a cold war standoff with the former Soviet Union.

Following a series of nuclear embarrassments in 2008 — including the inadvertent transport of six nuclear-tipped missiles on a B-52 bomber, whose pilot did not know they were aboard when he flew from Minot to Barksdale — then-defense secretary Robert Gates fired the top two air force officials.

Kowalski's command was created in late 2009 as part of an effort to fix what was broken in the nuclear force. In Tuesday's interview he said he is encouraged that inspections after 2009 began finding an increasing number of problems at the ICBM wings, followed by a decrease since 2011. He said this tells him that the air force has come up with more rigorous, effective means of inspecting, and that they are spurring change.

"This is a difficult inspection," he said, so occasional failures do not point to a systemic failure to adhere to safety and security regulations.