Israel British and American authorities have spotted multiple drones flying over four U.S. military bases in England in recent days and are investigating. Britain has deployed dozens of troops around the bases, concerned that the drone overflights could be an act of deception or sabotage by adversaries such as Russia.

In a statement issued on Wednesday (November 27), the U.S. Air Force in Europe said, “Since November 20, small drones have been spotted near and over RAF Lakenheath, RAF Mildenhall, RAF Feltwell and RAF Fairford.” The statement said the number of drones spotted has been fluctuating and the size and configuration vary.

“So far, base leaders have determined that none of these intrusions have affected base residents, facilities or assets. The Air Force is taking all appropriate measures to protect the above bases and their residents,” the statement said.

RAF Lakenheath, located in eastern England, is home to the U.S. Air Force’s 48th Fighter Wing, the cornerstone of its combat capabilities in Europe and the base for several F-35 stealth fighters and other aircraft. Four US B-52 strategic bombers are currently stationed at RAF Fairford in western England.

The Pentagon said this week that it was actively monitoring the situation. “The bottom line is, this is something we take seriously. We are continuing to investigate. But as of now, there has not been any significant impact on the mission,” Pentagon spokesman Maj. Gen. Patrick Ryder told reporters on Tuesday (November 26). He added that small drones “are now becoming relatively common throughout the region.”

Britain has deployed about 60 troops around the US base. British Defense Secretary Maria Eagle said they are using “multi-layered force protection measures.”

Eagle told lawmakers on Wednesday: “We will ensure that anyone we catch engaging in this behavior will be punished to the full extent of the law.”

David Dunn, a professor of international politics at the University of Birmingham in the UK, who has written extensively about the dangers of drones, said the nature of the drone sightings indicated that they were not operated by amateurs.

“What’s particularly concerning in this case is that there were actually several different sizes of drones. This appears to be a coordinated and planned activity. Most obviously, these are disruptive practices that actually force airports to change how they operate and suspend air operations,” Dunn told Voice of America (VOA).

Drones can also be used for other purposes.

“They can gather intelligence on how many aircraft are flying, where they’re based, how they’re flying. And they can actually do that for individuals as well,” Dunn said.

According to Dunn, drones were spotted over RAF Feltwell, which primarily serves as accommodation for American service members — a “particularly sinister” development.

“Because in an age of highly advanced fifth-generation aircraft that can fly stealthily, stealthily in the electronic spectrum when they fly — and are highly protected in hardened aircraft shelters at air bases — the most vulnerable part of the whole system is actually the crew.”

“So if you can determine where they’re staying by tracking them home to their accommodation, you can determine where you can actually break the weakest part of that chain,” Dunn told VOA.

The Times of London reported that authorities have not ruled out Russia as the culprit. Dunn said there is evidence that Moscow is trying to step up hybrid attacks, which means a non-military form of warfare that could still be destructive.

“Whether it’s sabotaging undersea cables, or flammable packages sent to Birmingham, the city where I live – a flammable package was found at Birmingham Airport. There was another example of a warehouse burned down that was storing supplies destined for Ukraine. These things often seem to be done by third parties, and the Russian government seems to have hired criminals in the UK,” Dunn said.

As of publication, the Russian Embassy in London has not responded to VOA’s request for comment. Moscow has previously denied involvement in hybrid attacks against the West.