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.

Unidentified drones have been spotted over U.S. bases in the United Kingdom used by American forces for a week, Fox News has confirmed.

US officials are monitoring drone sightings near three bases in England. 60 British personnel are aiding the investigation.

The Air Force didn’t identify who was behind the incursions, but said officials determined there was no impact on residents or critical infrastructure.

RAF Mildenhall is primarily home to the USAF's 100th Air Refuelling Wing; RAF Lakenheath is home to USAF F-35A and F-15E fighter jets; and RAF Feltwell is mainly concerned with logistics and provides housing for military personnel.

It is not ruled out that Russia may be behind it. The sightings began when London authorized Ukraine to use ballistic missiles on Russian soil. Sixty electronic warfare specialists from the Royal Air Force (RAF) have been sent to the American bases on British soil due to the suspicious presence of drones near the military facilities since last week.

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.