Sirens sounded at RAF Lakenheath and nearby RAF Feltwell after suspected unauthorised drone activity was reported over the Suffolk airbase.
Residents said alarms began shortly after 5.30pm on Friday, followed by a loudspeaker announcement in an American accent telling people to remain indoors and take shelter.
The US Air Force said its security personnel had responded to the reported drone activity at RAF Lakenheath. It said no personnel, aircraft or infrastructure had been affected.
“We continue to work closely with our host nation counterparts to ensure the safety and security of our US service members, US and host nation civilians, contractors, and all of our families,” a spokesperson said.
Nicole Barraclough, 22, who lives near RAF Feltwell, said she was at home with her toddler when the alarms began.
“At around half 5 I was in my living room with my toddler and I heard what sounded like war sirens,” she said.
“At first I thought it was a car playing weird music past my house. Unfortunately though the sound continued. It continued for five minutes.
“We thought maybe it was a drill. Then we heard an American male voice from the speakers telling people to remain indoors and shelter.”
Ms Barraclough said the incident had left her “shaken up”. Videos recorded by residents showed the sirens sounding near the bases after darkness had fallen.
West Suffolk MP Nick Timothy said he had contacted officials at RAF Lakenheath amid speculation about the incident and was told there was “no threat to the public” and that the situation had been resolved.
He urged residents to continue reporting anything unusual or suspicious to the authorities, and called on the Ministry of Defence and the US Air Force to communicate as openly as operational security allowed.
The incident comes days after the US military moved at least 12 aircraft from RAF Mildenhall in Suffolk following a security threat linked to Iran. Nine US Air Force refuelling aircraft were flown to bases in Britain, Germany and Spain on November 9, while three V-22 Ospreys were moved to RAF Lakenheath.
Security at Mildenhall and Lakenheath was subsequently raised to Force Protection Condition Charlie, the second-highest threat level, used when intelligence indicates a terrorist attack is likely.
US bases in Britain have been subject to heightened security since an incident at RAF Fairford in September. Police found petrol but no explosives while searching vans near the Gloucestershire base after men approached it in white vans during the night.
Two men were charged last month in connection with that incident. Prosecutors allege that one had attempted to assist Iranian intelligence, although Tehran has denied any involvement.
The US Air Force has encouraged people to report unusual or suspicious activity to local authorities. The Ministry of Defence has been contacted for comment.
