LONDON (Reuters) - 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. The Pentagon said that there had not been “any significant mission impact.”

The U.S. Air Force reported several incursions occurred in the past week at four bases where the American forces operate.

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.

US Air Force in UK says a small number of drones spotted flying over bases in eastern England.

A number of unidentified drones have been spotted over three United States Air Force bases in England over the last six days, U.S. officials said on Tuesday. "Small unmanned aerial systems were spotted in the vicinity of and over RAF Lakenheath,

The U.K. sent military personnel to help jam and track the drones after sightings.

Drones have been spotted near three military bases in Britain but they have not been identified as hostile, the US air force (USAF) says. In a statement on Tuesday, the air force said the sightings occurred on Monday and overnight.

Unidentified drones have been spotted again over three British air bases used by the U.S. Air Force, officials confirmed Tuesday. The three bases are all located within the same area of eastern England between Cambridge and Norwich,