WASHINGTON - Ten US Air Force officers are being investigated for alleged illegal drug possession, the Air Force said on Friday, a day after the probe surfaced as Defense Secretary Chuck Hagel was visiting a base to rally airmen in the US nuclear force.

Lieutenant Colonel Brett Ashworth, an Air Force spokesman, said nine lieutenants and a captain at six bases in the United States and Britain were implicated in the investigation.

Ashworth said the investigation began with two officers at Edwards Air Force Base in California and "expanded, based on contact with the officers in question regarding recreational drug possession." Edwards is the second-largest US Air Force Base and is central to new aircraft and weapons testing.

The probe has expanded to include officers at Malmstrom, Vandenberg, F.E. Warren and Schreiver Air Force bases as well as Royal Air Force Lakenheath in England, home to a US fighter wing, Ashworth said.

News of the investigation emerged on Thursday after the Air Force suspended the security clearances of two missile launch officers in a drug probe at Malmstrom Air Force Base in Montana. The base is one of three responsible for the United States' 420 nuclear-armed intercontinental ballistic missiles.

The Air Force action came as Hagel was at a sister nuclear missile base - F.E. Warren in Cheyenne, Wyoming - preparing to rally troops responsible for maintaining and staffing the launch sites.

Vandenberg Air Force Base in California is home to the Air Forces space and missile testing facilities, and Schreiver in Colorado is home to its Space Warfare Center and Ballistic Missile Defense Organization.