WASHINGTON — An Air Force unit that operates one-third of the nation’s land-based nuclear missile force 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 Air Force’s nuclear force told The Associated Press today.

The failure was recorded by the 341st Missile Wing at Malmstrom Air Force Base, Mont., which is responsible for 150 Minuteman 3 nuclear missiles that stand on 24/7 alert for potential launch against targets around the globe.

Lt. Gen. James M. Kowalski, commander of Air Force Global Strike Command, which is responsible for the Minuteman 3 force as well as nuclear bombers, said the 341st wing failed a “small team exercise” as part of a broader inspection. The exercise failure meant the whole inspection was a failure, he said.

“This unit fumbled on this exercise,” Kowalski told the AP by telephone from his headquarters at Barksdale Air Force Base, La.

He said those who failed the exercise would be retested within three months.