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…