Counter-terrorism police is understood to be leading the investigation into a major incident near a military airbase after several men were arrested on suspicion of explosives offences.
The police operation near the village of Whelford in Gloucestershire was not intelligence-led, meaning investigators had not known about any potential plot in advance.
Villagers have been evacuated from their homes and the Army's bomb disposal team is currently examining a number of vehicles at Royal Air Force Fairford in Gloucestershire.
The base also hosts the United States Air Force (USAF) and has been used to launch operations against Iran during the Middle East conflict.
A large emergency services presence was seen in the area, including a Hazardous Area Response Team.
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.”
Defence Secretary Wes Streeting confirmed the incident 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.”
Prime Minister Andy Burnham is receiving updates on the situation.
A government spokesperson added: “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”.
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 (RIAT), though this year’s event in May was cancelled because of the military operations.
Security measures at the site in southwest England have significantly increased with roadblocks and armed police preventing access and emergency vehicles parked nearby, Reuters reported on Sunday.
A spokesperson for the US Air Force said its personnel based at RAF Fairford “remain vigilant”.
“For operational security reasons we will not discuss specific force protection measures,” the spokesperson added.
“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.
“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 circumstances around the current incident are not yet known. In July, the British government insisted it “stands ready 24/7 to defend itself” after Iran said UK airbases used by the US were considered “legitimate targets”.
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