According to a spokesperson for the European Command, multiple drones were observed at three military bases, RAF Lakenheath, RAF Mildenhall in Suffolk, and RAF Feltwell in Norfolk, between November 20 and 22. RAF Lakenheath was once a US nuclear weapons storage facility during the Cold War, the US Air Force (USAF) confirmed.

The military said the number of mysterious drones "fluctuates and changes in size and configuration" and it is unclear whether they pose a hostile threat.

The Pentagon declined to comment when asked whether any air defenses were used against the drones. The agency said it was actively monitoring the situation to ensure no intrusions affected base personnel or critical infrastructure.

"In order to protect operational security, we do not discuss our specific measures. We continue to monitor the airspace and are working with host nation authorities and our partners to ensure the safety of base personnel, facilities and assets," a Pentagon spokesperson said.

The Ministry of Defence also declined to comment on the specific “security procedures” in place, with a spokesman saying the military took the threats “seriously” and maintained “robust measures” at key defence sites.

RAF Lakenheath was one of three US nuclear weapons storage sites in the UK during the Cold War. It housed around 110 US nuclear warheads as of 2008.

According to previous media reports, the Pentagon is preparing the base to store nuclear weapons once again. The US military requested $50 million for a new storage facility at RAF Lakenheath last year in a request to Congress.