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.

A spokesperson for the United States Air Force said in a statement following the incursions that the drones "fluctuated and they ranged in size [and] configuration"

Since the first small unmanned aerial system was first spotted on Nov. 20, the number of drones has fluctuated and there have been a range of different sizes and configurations, said the U.S. Air Forces in Europe spokesperson.