The US Air Force gave a nuclear missile unit an "unsatisfactory" rating, according to a statement released Tuesday, in a setback for strategic forces after a similar incident in May.

An inspection conducted August 5-13 assessed the 341st Missile Wing's "ability to execute operations while complying with nuclear surety standards."

The Air Force said the unit made "tactical-level errors during one of several exercises," without providing further details about what went wrong.

"A failed inspection does not mean that the safety of the nation's nuclear arsenal is at risk," Lieutenant General Jim Kowalski said.

The unit, housed at Malmstrom Air Force Base in Montana, controls 150 Minuteman III intercontinental ballistic missiles, which comprise one-third of America's ICBMs.

The incident marks the second time this year that the Air Force has experienced trouble with a unit that controls nuclear missiles.

In early May, 17 officers based in Minot, North Dakota, were stripped of their certification after a poor assessment of their launch operations.

Although it is not uncommon for an officer to lose accreditation and be returned to training, 17 officers losing accreditation at the same time is unprecedented, Lieutenant Colonel John Dorrian, a spokesman for the Air Force, told AFP.

Malmstrom Air Force Base will be inspected again within 90 days.