Swarms of small drones have been spotted flying over joint U.S.-U.K. military bases in the United Kingdom for nearly a week, prompting the U.K. to launch a criminal investigation.

The drones were spotted at four U.K. military bases where U.S. forces operate and house their American F-15 Strike Eagle and F-35 fighter jets. Military officials express concern over what appears to be an effort to test security at the four bases, which include the RAF Lakenheath, RAF Mildenhall, RAF Feltwell, and RAF Fairford.

As it entered a German port, another drone was seen Friday flying near the Royal Navy’s flagship aircraft carrier, HMS Queen Elizabeth.

The source of the drones has not been identified, but a U.S. military official told Fox News that the drones are more sophisticated than those that can be purchased off the shelf. Eyewitnesses described the drones as small quadcopters and octocopters.

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The U.K. Defense Ministry is working with the U.S. military and local police to investigate what the drones may be doing and who may be responsible for them.

“We take any safety issue seriously and maintain robust measures at Ministry of Defense sites,” Defense Minister Vernon Coaker told Parliament. “This includes counter-drone capabilities.”

He added that persons convicted in connection with the drone incursions could face up to 14 years in prison under the U.K.’s national security laws.

Although the intent of the drone operators is unclear, it has been suggested that the repeated incursions may be a signal by a state actor, such as Russian President Vladimir Putin. The drone incursions began shortly after Ukraine escalated its attacks on Russian targets using weapons supplied by the U.S. and Britain. Putin stated that his country had the right to strike nations that allow their weapons to be used against Russia.

“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,” the U.S. Air Forces Europe said in a published statement.

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