The U.S. Air Force moved aircraft from RAF Mildenhall in eastern England after receiving a threat, a U.S. official told the Associated Press. The aircraft included aerial refueling tankers and Ospreys. Some were based at Mildenhall and others were there on rotation.

CBS News separately verified departures of KC-135 tankers using public flight records. A U.S. official told CBS that there were credible threats against bases in Britain and did not identify who was behind them. The U.S. officials who discussed the threat publicly through news outlets did not describe it in detail.

A U.S. Air Force in Europe official told AP that the military had taken steps to protect U.S. and British personnel and their families. The official declined to detail the aircraft movements or specific security measures. Britain’s Defense Ministry said aircraft regularly move according to operational needs and declined to discuss another country’s activities.

Mildenhall is home to the 100th Air Refueling Wing. Its KC-135 Stratotankers refuel U.S. and allied aircraft in flight, allowing them to stay airborne longer without landing. The wing supports operations across Europe and Africa. Its Air Force fact sheet lists contingency missions, special operations support and training among its duties. RAF Mildenhall has hosted the 100th Air Refueling Wing since 1992. The Air Force has not said when the relocated aircraft will return.