Unidentified drones have been spotted again over three British air bases used by the U.S. Air Force, officials confirmed Tuesday.

They were first seen last week over Royal Air Force Lakenheath, Royal Air Force Mildenhall and Royal Air Force Feltwell, the U.S. Air Forces in Europe said in a statement.

The "small unmanned aerial systems" varied in number, size and configuration, it added.

The three bases are all located within the same area of eastern England between Cambridge and Norwich, about two hours north of London.

"To safeguard operational security, we do not discuss specific force protection measures, however we retain the right to protect our installations," the U.S. Air Force said.

On Tuesday, the U.S. Air Force confirmed the situation was "still ongoing."

"I can confirm the situation is still ongoing and our units continue to monitor the airspace and are working with host-nation authorities and mission partners to ensure the safety of base personnel, facilities, and assets," the spokesperson said.