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.

“[We] can only confirm that the number [of drones] fluctuated and varied between the bases over the night,” a USAF in Europe spokesperson said in a statement.

The statement added that there had been no impact on the military bases’ residents or infrastructure, adding 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 US official who spoke to the Reuters news agency on the condition of anonymity said the drones did not appear to be operated by people who fly drones as a hobby because the aircraft appeared to be too coordinated.

The official added that despite the initial analysis, it was too early to say who might be responsible.

‘Sinister’ plot

A spokesperson for the United Kingdom Ministry of Defence said the country takes the “threats seriously” and was supporting the USAF.

Britain’s PA Media news agency also reported that British troops were drafted to help the USAF uncover who was responsible for the drones.

According to the British newspaper The Times, which quoted a UK military source, there were concerns the drones could be part of a “sinister” plot.

Since Wednesday, US military bases at RAF Mildenhall and RAF Lakenheath in Suffolk and RAF Feltwell in Norfolk have reported sightings of drones in the area.

Mildenhall is home to the USAF’s 100th Air Refuelling Wing. Lakenheath is home to its F-35A and F-15E fighter jets, and Feltwell is home to USAF logistics and military personnel.