Unidentified drones have been observed over three U.S. Air Force bases in England during the past six days, a spokesperson for the USAF confirmed on Tuesday.

The U.S. Air Force (USAF) said it was monitoring the airspace over East Anglia bases RAF Lakenheath, RAF Mildenhall and RAF Feltwell after the drones were seen overnight.

USAF said the number of sightings "fluctuated and varied between the bases" but they have not been identified as hostile.

It comes days after the USAF confirmed that a small number of "unmanned aerial systems" were spotted in the vicinity of and over the three bases in eastern England between Wednesday and Sunday.

U.K. authorities are working to ensure the safety of the sites.

USAF has not identified who is believed to be behind the incidents.

In a statement, a spokesperson for USAF in Europe said: "We can confirm there were sightings yesterday (Monday) during night-time hours and can only confirm that the number fluctuated and varied between the bases over the night.

The three bases, two in Suffolk and one in Norfolk, are leased by the USAF from Britain.

A spokesperson for Britain's Ministry of Defence said they "take threats seriously and maintain robust measures" at defense sites.

"We are supporting the U.S. Air Force response," the spokesperson added.