The Air Force quietly modified its procedures for loading launch codes in Minuteman nuclear missiles after discovering three years ago that some of the weapons had been accidentally rendered impotent.

The service, responding to a published report Monday, acknowledged that some Minuteman 3 missiles at Malmstrom Air Force Base in Montana could not have been fired in 1986 if the United States had gone to war because they didn't have the right launch codes inside their internal computers.The Air Force said the precise cause of the mistake was never determined, but "inadvertent personnel error was the most likely cause."