LONDON (AP) — The U.S. Air Force said Tuesday that a number of small drones have been detected near four bases in England that are used by American forces.

The flights, which began Nov. 20, were spotted over or near RAF Lakenheath, RAF Mildenhall and RAF Feltwell in eastern England as well as RAF Fairford in southwestern England. The drones were actively monitored.

The Air Force hasn’t identified who is believed to be behind the incursions. Unspecified mitigation measures are underway.

“The number of systems has fluctuated, and they have ranged in sizes and configurations," U.S. Air Forces Europe said in a statement. ”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.″

Lakenheath is home to the 48th Fighter Wing, which the U.S. Air Force describes as the foundation of its combat capability in Europe. Mildenhall hosts the 100th Air Refueling Wing, and Feltwell is a hub for housing, schools and other services. Fairford is home to the 501st Combat Support Wing Headquarters and the 420th Air Base Squadron.

“To date, installation leaders have determined that none of the incursions impacted base residents, facilities or assets,″ the Air Force said. ”The Air Force is taking all appropriate measures to safeguard the aforementioned installations and their residents.″

Though it is unclear whether the drones had hostile intent, the most recent incidents come at a time which saw the most significant escalation of hostilities in Ukraine since Russia’s full-scale invasion nearly three years ago.

Ukraine struck targets inside Russia with intermediate range missiles supplied by the U.S. and Britain after U.S. President Joe Biden authorized use of the weapons. Soon after, President Vladimir Putin said his country had the right to strike nations that allow their weapons to be used against Russia.