Around 60 British troops, including counter-drone specialists, have been deployed to help defend three air bases used by the US Air Force in the UK after a spate of "weird" drone sightings.
The military personnel will also support efforts to find out who is operating the unmanned aerial vehicles amid fears the mysterious sorties might be part of some "coordinated" operation.
Civilian police, supported by Ministry of Defence police, have launched an investigation into the drone activity.
"It seems awfully coordinated," a defence source said after a number of drones were again spotted overnight over RAF Lakenheath and RAF Mildenhall, in Suffolk, and RAF Feltwell in Norfolk.
The sightings came after similar drone movements over the same sites between Wednesday and Friday last week - a highly unusual occurrence.
A second defence source said about the drone activity: "It's odd. Very weird."
A spokesperson for the US Air Forces in Europe confirmed a number of drones had been spotted on Monday "during nighttime hours".
"The number fluctuated and varied between the bases over the night," the spokesperson said.
Asked whether the drones overnight and last week had been deemed hostile, the spokesperson said: "Since November 20 there has been no impact to residents or infrastructure and [they] have not been identified as hostile.
"However they are still continuously being monitored to ensure the safety and security of the installations."
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A Ministry of Defence spokesperson said: "We take threats seriously and maintain robust measures at defence sites. We are supporting the US Air Force response."
This is understood to include the deployment of around 60 military personnel, including members of the RAF Regiment's Force Protection Wing, with specialist skills in countering drones to help protect the US bases and try to find out who is responsible for operating the unmanned aerial vehicles.
The RAF troops operate the ORCUS counter-uncrewed air system, which has the ability to "detect, track, identify and, if necessary, defeat hostile drones", according to the RAF website.
US military units on Tuesday were still monitoring the airspace over the three bases. RAF surveillance capabilities have also been deployed.
"I can confirm the situation is ongoing and 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 US Air Forces in Europe spokesperson said.
"To safeguard operational security, we do not discuss specific force protection measures, however we retain the right to protect our installations."
(c) Sky News 2024: British forces deployed after drones spotted again over three US air bases in UK
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