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"

Drones over the bases in Britain ranged in size and configuration, a U.S. Air Forces in Europe spokesperson said.

Unidentified drones have been spotted flying over three bases used by the United States Air Force in Britain, the two countries have confirmed.

The USAF, which uses the bases, said it was unclear at this stage whether the drones were considered hostile. It also declined to comment on whether any defence mechanisms were used, but said it retained "the right to protect" installations.

A mystery drone has been spotted following a British aircraft carrier at sea after unmanned aerial vehicles were seen hovering over three air bases in England.

Unidentified drones have been spotted over three US airbases in Britain, the United States Air Force (USAF) has confirmed. The sightings between 20 and 22 November were of a number of “small unmanned aerial systems” over RAF Lakenheath and RAF Mildenhall,