Unidentified drones have been spotted over joint U.S.-U.K. bases in the United Kingdom for nearly a week.

Fox News’ Jennifer Griffin reports that four U.S. military bases in the U.K. that house the American F-15 Strike Eagle and F-35 fighter jets have been targeted by “swarms of small drones” since Wednesday, Nov. 20. 

Military officials say they are “alarmed” at what appears to be a coordinated effort to test security at RAF Lakenheath, RAF Mildenhall and RAF Feltwell in eastern England, as well as RAF Fairford in southwestern England.

The U.K. military has sent around 60 personnel to protect the bases being targeted by multiple drone incursions.

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A senior U.S. official told Fox that the drones do not appear to be those of “hobbyists.” Witnesses say they are small quadcopters and octocopters and are more sophisticated than those that can be bought off the shelf. 

Pentagon press secretary Maj. Gen. Pat Ryder said earlier Tuesday that none of these drone incursions have impacted the base residents, the facilities or U.S. assets on the bases. 

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“We, of course, always maintain the right to defend ourselves if we deem something to be a threat,” Ryder said. 

U.S. officials have not revealed the origin of the drones, but the coordinated nature suggests an aggressive state actor that began a day after Ukraine used long-range American ATACMs to target Russia after President Biden lifted restrictions on the use of longer-range missiles. 

A U.S. Air Force McDonnell Douglas F-15E Strike Eagle of the 492nd Fighter Squadron (Madhatters) 48th Fighter Wing applies the air brake at RAF Lakenheath on Dec. 15, 2021. (Jon Hobley/MI News/NurPhoto via Getty Images)

Soon after, President Vladimir Putin said his country had the right to strike nations that allow their weapons to be used against Russia.

The U.K. Defense Ministry said it is working closely with U.S. and local police to find out what these drones are doing and whether a foreign adversary is involved. 

“The number of systems has fluctuated, and they have ranged in sizes and configurations,” U.S. Air Forces Europe said in a statement. “Our units continue to monitor the airspace and are working with host-nation authorities and mission partners to ensure the safety of base personnel, facilities and assets.”

A U.S. Air Force Lockheed MC-130J Commando II Hercules (STRIX44) of the 67th Special Operations Squadron (Night Owls) approaches RAF Mildenhall in England on Oct. 5, 2020. (Jon Hobley/MI News/NurPhoto via Getty Images)

The incursion of drones comes a month after Fox first reported that surveillance drones flew for 17 days near Langley Air Force Base in Virginia last December. Those drones were large but pose a similar challenge to base commanders. 

Lakenheath is home to the 48th Fighter Wing, which the U.S. Air Force describes as the foundation of its combat capability in Europe. Mildenhall hosts the 100th Air Refueling Wing, and Feltwell is a hub for housing, schools and other services. Fairford is home to the 501st Combat Support Wing Headquarters and the 420th Air Base Squadron.

The Associated Press contributed to this report.