Unidentified drones over U.S. air bases in the United Kingdom reportedly prompted British senior officials to hold secret meetings to discuss the emergency, which could be the work of an enemy state.
U.K. military officials have expressed alarm that it has been impossible to find the pilots responsible for controlling the drones and have not ruled out that the aerial objects are the work of a hostile state, The Times of London reported.
The drones involved sophisticated technology that prevented authorities from tracking them or identifying those responsible. None have been captured, nor have they been affected by electronic countermeasures such as jamming, the outlet reported.
Investigators believe the drones were controlled in real time by live operators and not programmed.
Authorities have ruled out the work of hobbyists.
"They're very sophisticated, very fast. This is not the work of hobbyists but no one is confident of attribution at the moment," one British official told the Times.
The U.K. situation is similar to that seen over parts of New Jersey, New York, and neighboring states, where drones have been seen since Nov. 18.
At least two COBRA (Cabinet Office Briefing Room A) meetings have been held, the Times reported.
COBRA meetings, gatherings of senior government officials, often are held in time of national crisis, according to the Security Journal United Kingdom. The purpose of a COBRA meeting is to ensure that the government can respond quickly and effectively to crises, with all relevant information and decision-makers in one room.
U.S. Navy members have joined British military police to investigate the drones seen over Royal Air Force (RAF) Lakenheath, RAF Mildenhall, RAF Fairford, and RAF Feltwell.
Facilities to house nuclear weapons are under construction at Lakenheath, the Times reported.
British and American intelligence services have probed whether Russia's military intelligence agency, GRU, was responsible.
"It seemed to us that it was plausible the Russians were behind the flights but it could have been non-state groups," a U.S. intelligence source told the outlet. "There has been no firm evidence that Russians were involved."
U.S. military officials say the drones, flown over the bases in late November and early December, varied in size and configurations.