"We can confirm there were sightings yesterday (Monday) during nighttime hours and can only confirm that the number fluctuated, and varied between the bases over the night," said a USAF in Europe spokesperson, according to a statement sent to AFP by the 100th Air Refueling Wing from its Mildenhall base.

Mildenhall, in eastern England, is one of three bases where drones have been spotted intermittently since November 20.

The other two are the nearby RAF Lakenheath and RAF Feltwell.

The USAF said on Monday that "small unmanned aerial systems were spotted in the vicinity of and over Royal Air Force Lakenheath, RAF Mildenhall and RAF Feltwell between Nov. 20 and Nov. 24."

The drones ranged in sizes and configurations, it added.

In Tuesday's update, the USAF said that there had been no impact to the bases' residents or infrastructure, and that the drones "have not been identified as hostile".

"However they are still continuously being monitored to ensure the safety and security of the installation," it added.

A spokesman for the UK's Ministry of Defence said that it took "threats seriously" and was supporting the USAF response.