The U.S. Air Force says more unexplained drones have been sighted near major British military bases used by American forces.

The U.S. Air Force announced several unmanned aerial systems were detected over three bases it has used in the United Kingdom last week. The drones, which ranged in sizes and configurations, were spotted last week between Wednesday and Friday “in the vicinity of and over” Royal Air Force Lakenheath,

The drones were detected between Wednesday and Sunday, near Royal Air Force Lakenheath, Royal Air Force Mildenhall and Royal Air Force Feltwell, all three of which are in eastern England. The bases house thousands of U.S. service members, primarily Air Force personnel.