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.

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.

Unidentified drones have been spotted flying over four bases used by the United States Air Force in Britain, prompting the British government to call in military staff to help protect the sites. The U.

The British military has increased security around four air bases in the country that are used by U.S. forces after "a number" of drones were spotted near them.