Counter-terrorism police are leading the investigation that saw several men arrested on suspicion of explosive offences near an RAF base, the Press Association understands.
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.
Evacuations and emergency response
Villagers have been evacuated from their homes near RAF Fairford, which is used by the United States Air Force (USAF), and the Army's explosives experts are examining vehicles, police said.
A major incident was declared by police and a large emergency services presence was seen in the area, including a Hazardous Area Response Team.
Aerial footage showed a bomb disposal robot inspecting three white vans, two of which had open rear doors. Large black items were scattered around.
Prime Minister Andy Burnham is receiving updates on the situation.
Residents describe the incident
Julian Tudsbery, who has lived in Whelford for 30 years, said a resident had reported three "suspicious vans" in the village in the early hours of Sunday morning.
"Evidently, they phoned the airbase police, who didn't answer the phone," he said. "It just rang out so she dialled 999."
He said he was evacuated at around 7am, adding: "(The police) were then banging on the door, so I went downstairs in my dressing gown.
"There were several police, three police cars in the drive, and they said you need to evacuate, there's been a major incident, said he couldn't elaborate."
Mr Tudsbery said evacuated villagers have been asked if they have places to stay tonight.
About 40 or 50 people were asked to leave their homes for Cirencester Leisure Centre and about 20 remained there at about 1pm, another resident said.
Meanwhile a woman living on the base's perimeter described an apparent increase in military security in recent weeks.
The woman, who asked not to be identified, told the Press Association: "In the prior few weeks they have put heavy, very heavy, vehicles across all gates."
She described them as "very big tractors and things like that" parked behind crash-gates into the base, adding: "So obviously they're taking security seriously".
"Great big concrete walls" have also been placed behind some gates, including on Dunfield Loop and Whelford Road, the resident, who has lived there for about 50 years, told PA.
Threat context and official statements
The arrests come amid heightened concerns around the threat from hostile states to the UK, with Russia and Iran among those that have used proxies to commit crimes on their behalf on British soil.
In March, officials said the number of national security cases involving activity by hostile states including Iran increased 50% in the six months to December 2025.
Investigators will be working to analyse any devices seized from those arrested to establish details of any plot and any potential motivation.
Gloucestershire Police said in a statement that several men had been arrested on suspicion of offences under the Explosives Act.
"The Army's specialist explosive ordnance disposal team are currently examining a number of vehicles," the force said.
"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.
RAF Lakenheath in Suffolk is on Charlie alert, the second-highest threat level that can be issued by the US Department of Defence.
US airbase Lakenheath confirmed its current threat level to the Press Association.
This comes after reports that RAF Fairford has been raised to the highest threat level, Delta.
Force Protection Condition (FPCon) Charlie "applies when an incident occurs or intelligence is received indicating some form of terrorist action targeting personnel or facilities is likely", according to a US government website.
Delta "applies in the immediate area where a terrorist attack has occurred or when intelligence has been received that terrorist action against a specific location or person is imminent".
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."
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."
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.
