Recent sightings of drones at night over airbases in the United Kingdom used by the United States Air Force (USAF) have prompted fears that unknown actors may be conducting hostile reconnaissance of the facilities.

“Small unmanned aerial systems continue to be spotted in the vicinity of and over Royal Air Force Lakenheath, RAF Mildenhall, RAF Feltwell and RAF Fairford since November 20,” a spokesperson for the USAF in Europe told Recorded Future News.

The airbases in eastern England are owned by the British Ministry of Defence (MoD) but exclusively used by the USAF, with the bases home to numerous operational fighter jets, including the F-35 Lightning II, as well as facilities for logistics.

“To date, installation leaders have determined that none of the incursions impacted base residents, facilities or assets. The Air Force is taking all appropriate measures to safeguard the aforementioned installations and their residents,” the spokesperson added.

The drones seen both last week and on Monday night ranged in size and configuration, and the number “fluctuated, and varied between the bases over the night” the spokesperson said.

While the nature of the flights is not yet confirmed, they are taking place while security concerns across Europe are heightened over Russian activities intended to undermine and sabotage support for Ukraine.

The repeated nature of the flights were described as “awfully coordinated” by a defence source who spoke to Sky News which also reported that British counter-drone specialists had been deployed to defend the three bases. 

A spokesperson for the MoD said: “We take threats seriously and maintain robust measures at defence sites. We are supporting the US Air Force response.”

Similar sightings have also occurred over airbases in the United States, including the Edwards Air Force Base and Langley Air Force Base, with USAF Gen. Mark Kelly telling The Wall Street Journal that the flights had the Pentagon stumped about who could be behind them and how to respond. 

In August, the alert was sounded in Norway when drones were spotted flying over a facility used by arms manufacturer Kongsberg Group which produces anti-aircraft defences used by the Ukrainian military.

A spokesperson for Kongsberg told Norway’s TV2 that while the company was aware it was an intelligence target, it was unsure of whether the incident was hostile or caused by hobbyists.

Alarm has been growing across Europe over Russian-directed sabotage operations, with security agencies and governments increasingly warning about the threat posed by saboteurs.

Earlier this year, the North Atlantic Council, NATO’s political executive, announced that allies were “deeply concerned about recent malign activities on Allied territory, including those resulting in the investigation and charging of multiple individuals in connection with hostile state activity.”

Several arrests have taken place in the United Kingdom of individuals accused of working on behalf of the Russian intelligence services for sabotage and espionage purposes.

The British government cited the spying case when it announced earlier this year what it called a “major package of measures to target and dismantle Russian intelligence gathering operations” in the country, including removing the diplomatic premises status of an estate in the country allegedly used by Russian intelligence officers to meet agents.