Sirens sounded at RAF bases on Friday amid reports of suspected drone activity.
Residents near RAF Lakenheath in Suffolk and neighbouring RAF Feltwell in Norfolk said they heard the alarms during the evening.
Local people reported hearing what sounded like “war sirens” at about 5.30pm, followed by an announcement over loudspeakers. An American-accented voice instructed residents to remain indoors and take shelter.
The incident came only days after the US military evacuated at least 12 aircraft from RAF Mildenhall in Suffolk because of a security threat linked to Iran.
Disturbing footage recorded by residents near the bases showed sirens echoing across gardens in the darkness.
The US Air Force confirmed that the alarms at RAF Lakenheath were triggered by “suspected unauthorised drones”.
A spokesperson said: “This evening, USAF Security Forces personnel responded to suspected unauthorised drone activity over RAF Lakenheath.
“No personnel, aircraft or infrastructure were affected.
An F-15 fighter jet takes off from RAF Lakenheath in April 2018
A military vehicle stationed at RAF Lakenheath on Tuesday
A light armoured vehicle and military personnel at the entrance to RAF Lakenheath on September 27
“We continue to work closely with our host-nation counterparts to protect the safety and security of US service members, US and host-nation civilians, contractors and all of our families.”
The US Air Force urged anyone who noticed unusual or suspicious activity to report it to local authorities.
In a separate statement, a spokesperson said that “all measures” were being taken to protect the bases, aircraft and service members.
One resident near RAF Feltwell said she had been “shaken up” when the sirens began sounding near her home.
“At around half past five, I was in my living room with my toddler when I heard what sounded like war sirens,” Nicole Barraclough, 22, told the Daily Mail.
“At first, I thought it was a car passing my house while playing strange music. Unfortunately, the sound continued for five minutes.
“We thought it might be a drill. Then we heard an American male voice over the speakers telling people to remain indoors and shelter.”
West Suffolk MP Nick Timothy said he had contacted officials at RAF Lakenheath after speculation about the incident. He said he was told there was “no threat to the public” and that the situation had been resolved.
Disturbing footage recorded by residents near the airbases showed the sirens sounding across gardens in the darkness
Writing on social media, Mr Timothy urged residents to continue reporting anything suspicious or unusual to the authorities amid the heightened security situation.
He also called on the Ministry of Defence and the US Air Force to communicate as openly as operational security allowed.
Security has been increased at US bases in Britain since an incident at RAF Fairford in September that Prime Minister Andy Burnham said involved Iran.
Police discovered petrol, but no explosives, while searching vans near the base.
The discovery followed reports that a group of men had approached the base in white vans during the night.
Two men were charged last month in connection with the incident. Prosecutors alleged that one of them had attempted to assist Iranian intelligence in Britain.
Tehran denied any involvement, but the US withdrew all of its bombers from RAF Fairford the following month.
The US relocated bombers used to strike Iran out of Britain and moved large refuelling aircraft from the UK to Germany.
On November 9, nine US Air Force refuelling aircraft were moved from RAF Mildenhall to other bases in the UK, Germany and Spain.
Three V-22 Ospreys, which are used by American special forces, were also transferred to neighbouring RAF Lakenheath, where they can be kept in fortified hangars.
A former RAF chief described the sudden evacuation as “extraordinarily unusual”. It followed a similar withdrawal of bombers from RAF Fairford in Gloucestershire the previous month after an alleged Iranian terror plot was foiled.
The precise nature of the latest threat remains unclear and is being assessed by US officials, according to the Wall Street Journal.
However, RAF Lakenheath issued a drone warning last week, telling local residents and people living on the base to report sightings “immediately” to security staff or police.
Mildenhall and Lakenheath subsequently raised their security status to “Force Protection Condition Charlie”, the second-highest threat level. It is used when intelligence indicates that a terrorist attack is likely.
RAF Lakenheath is home to the US Air Force’s 48th Fighter Wing, known as the Liberty Wing. It has nearly 7,000 active-duty personnel, many of whom are based at the Suffolk station.
According to the wing’s website, the Liberty Wing is the largest US fighter operation in Europe. It includes four combat-ready fighter squadrons and supports thousands of military personnel and their families.
The British Ministry of Defence and Suffolk Police have been contacted for comment.