British military expertise has been brought in to help find those responsible for flying drones near US military bases.

About 60 RAF personnel have been sent to assist the US Air Force in its investigation, the BBC understands.

The drones were not considered hostile, the USAF said, adding the sightings had “fluctuated and varied between the bases” and had “ranged in sizes and configurations”.

The Ministry of Defence said: “We are supporting the US Air Force response.”

The news follows a number of recent reports of unmanned aerial vehicles spotted in the vicinity of RAF Lakenheath and RAF Mildenhall, both in Suffolk, and RAF Feltwell in neighbouring Norfolk.

Further drone activity had been reported overnight. The USAF said the vehicles were small in number and size.

A US Air Forces in Europe spokesman said: “We can confirm there were sightings yesterday during night-time hours and can only confirm that the number fluctuated and varied between the bases over the night.”

The spokesman added that since the first sightings on 20 November there had been “no impact to residents or infrastructure and they have not been identified as hostile”.

However, the drones were still being monitored “to ensure the safety and security of the installations”.

They continued: “We request individuals in the area to contact either local police or security forces if they see anything suspicious.”

Neither US nor UK officials have said who might be behind the recent drone activity.

But the BBC understands there were concerns a state actor might be involved.

In recent months intelligence officials have warned of increasing Russian sabotage operations against Western countries supporting Ukraine.

RAF Mildenhall is primarily home to the USAF’s 100th Air Refuelling Wing; RAF Lakenheath is home to USAF F-35A and F-15E fighter jets; and RAF Feltwell is mainly concerned with logistics and provides housing for military personnel.

A spokesman for the MoD, which owns the bases, said: “We take threats seriously and maintain robust measures at defence sites.

“We are supporting the US Air Force response.”

The USAF has not said who it believed to be behind the incidents.