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.

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.”

The government has warned that those behind recent drone incursions over US airbases will be "shown the full force of the law". Defence Minister Maria Eagle said it was "taking steps" after the mystery unmanned aerial vehicles (UAVs) were spotted over and around the three locations in Suffolk and Norfolk.