Villagers have been evacuated from their homes near an RAF base and several men have been arrested as the Army’s explosives experts examine vehicles, police said.
A major incident has been declared, with armed police manning a cordon near the village of Whelford in Gloucestershire, near RAF Fairford which is used by the United States Air Force (USAF).
A large emergency services presence was seen in the area, including a Hazardous Area Response Team.
Prime Minister Andy Burnham is receiving updates on the situation.
Gloucestershire Police said: “A number of properties are currently being evacuated in the Whelford area following the declaration of a major incident.
“It follows the arrest of several men on suspicion of offences under the Explosives Act.
“The Army’s specialist explosive ordnance disposal team are currently examining a number of vehicles.
“The local residents are being evacuated to a nearby leisure centre.
“A cordon is currently in place but we’d like to reassure people that we believe this incident is contained.
“We are working closely with our partners and are enacting the well-rehearsed plans we have in place.”
A spokesperson for the USAF said its personnel based at RAF Fairford “remain vigilant” but “for operational security reasons we will not discuss specific force protection measures.”
They said: “The 501st Combat Support Wing and our fellow UK-based Wings remain vigilant and will take appropriate actions to ensure the safety and security of our US service members, civilians, contractors, and their families.”
The 501st is “the only combat support wing in the USAF” and operates from 10 bases including Fairford, according to its website.
“We constantly assess a variety of factors that determine what measures we institute or modify to protect our installations, our people and their families”, the spokesperson added.
Defence Secretary Wes Streeting confirmed the incident near RAF Fairford is “being dealt with” and thanked the emergency teams who acted “so swiftly”.
He added: “It is exactly why our emergency response teams and Armed Forces are on standby 24/7.”
Meanwhile a Government spokesperson said “we are grateful to the emergency services, who have acted quickly to detain several suspects and put measures in place to keep the community safe.
“It would be inappropriate to comment further whilst investigations are ongoing.”
South Cotswolds Liberal Democrat MP Roz Savage described the situation as “worrying”.
She said on X: “I have contacted the police for further updates and I’ve been told that it is believed the incident is contained.
“I would like to thank them for the work they are currently doing.”
The Ministry of Defence has been contacted for comment.
In March this year, after then prime minister Sir Keir Starmer granted permission for “defensive” US action against Iranian missile sites from UK bases, four American bombers landed at RAF Fairford and the US started using British bases for “specific defensive operations”.
The airbase also hosts the annual Royal International Air Tattoo, though this year’s event in May was cancelled because of the military operations.
