U.S. troops in Europe reported a swarm of unidentified drones flying over several Air Force bases in the U.K. in the past week, a sighting that comes amid a growing mystery of UFO sightings near military installations.

Small unmanned craft were seen flying over four bases in England, the U.S. Air Force in Europe said in a statement. The Pentagon noted that there had not been “any significant mission impact.”

Unidentified drones are still being spotted over United States Air Force (USAF) bases in England on Wednesday, a USAF spokesperson said, confirming that a fourth base had also been targeted by the week-long airspace incursions.