Drones spotted over military bases in the U.K. that are used by the U.S.

LONDON — 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, the U.S. Air Force said Wednesday.

American officials were working with the British authorities to “ensure the safety of base personnel, facilities, and assets,” the U.S. Air Force in Europe said in a statement.

The U.K.'s Civil Aviation Authority has also imposed restrictions around the bases' airspace, a U.K defense minister told Parliament.

"The bottom line is it’s something that we’re going to take seriously," Pentagon spokesman Maj. Gen. Pat Ryder told reporters at a regular press briefing on Tuesday. "We’re continuing to look into it, but as of right now has not had any significant mission impact."

The drones, or small unmanned aerial systems, have been spotted "in the vicinity of and over the bases" since Nov. 20, a spokesperson said in a statement send via email from RAF Mildenhall, one of the four U.S bases.

The drones fluctuated in “size and configuration,” the Air Force said.

U.S. and British officials declined to say how many drones had been spotted, or who might be behind them.

The RAF Lakenheath, RAF Mildenhall and RAF Feltwell bases are in southeast England, about two hours north of London. The fourth base, RAF Fairford in Gloucestershire, is around 150 miles southwest of the other facilities.

The U.S. Air Force in Europe added they were working with the British authorities to “ensure the safety of base personnel, facilities, and assets.”

The spokesperson for U.S. Air Forces in Europe urged individuals in the area to contact local police or Security Forces in the U.K. if they notice anything suspicious, “including any drone activity.”

This article was originally published on NBCNews.com