Unidentified drones are still being spotted over United States Air Force (USAF) bases in England on Wednesday, a USAF spokesperson said, confirming that a fourth base had also been targeted by the week-long airspace incursions.

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

Small unmanned craft were seen flying over four bases in England, the U.S. Air Force in Europe said in a statement. The Pentagon noted that there had not been “any significant mission impact.”