Michael D. Shear and Amelia Nierenberg

Michael D. Shear reported from Liverpool, England, and Amelia Nierenberg from London.

5 are arrested on suspicion of terrorism near R.A.F. Fairford air base.

The British police arrested five men on Sunday on suspicion of preparing acts of terrorism after receiving a call around 1 a.m. about three suspicious vehicles headed toward an air force base used by the American military.

The base, R.A.F. Fairford, is primarily used by the United States Air Force to deploy some of its largest bombers and has been at the center of controversy this year over its role in the American-Israeli war against Iran.

It is not yet clear whether Sunday’s incident is connected to the conflict in the Middle East, but investigators are looking into whether there is any link to Iran or Russia. Both have been accused of orchestrating attacks on British soil.

In July, Iran’s Islamic Revolutionary Guards Corps issued a statement warning the British government that a decision to allow American bombers to conduct strikes against Iran would make “London responsible for” any consequences.

“Any base used to launch attacks on Iranian territory is a legitimate target for our forces,” it said.

When asked by reporters on Sunday about the arrests, President Trump said, “Working with Britain — it was an amazing job.” Appearing to refer to the suspects, he said: “We had them under investigation. They were looking to do big damage.”

Mr. Trump’s statement raised questions about whether the suspects had been under surveillance and, if so, how they were able to get so close to the base, possibly with explosives.

British officials said they had evacuated more than 85 homes in the rural village outside the air base in southern England early Sunday morning. On Sunday afternoon, hours after the arrests, aerial footage showed what appeared to be a bomb disposal robot continuing to operate near three white vans.

“A 400-meter cordon has been put in place around the vehicles while the army’s explosive ordnance disposal unit conduct examinations,” Vicki Evans, the deputy assistant commissioner of the British counterterrorism police, said in a statement.

Ms. Evans said that the counterterrorism police were leading the investigation. She said that the five men remained in custody under suspicion of “preparation of a terrorist act” and “suspicion of committing offenses under the Explosives Act.”

The men were not named, in line with British policing rules that grant public anonymity to suspects until they are charged.

Officials did not provide any further information about what the five men were suspected of doing, and the local police took no questions during a brief appearance in front of reporters.

After the arrests, the American Embassy in Britain issued a warning.

“U.S. citizens are encouraged to exercise caution and be mindful of personal security,” it said. “U.S. citizens are advised to follow instructions of local authorities for their personal safety and security.”

American B52 and B-1 bombers loaded with munitions have repeatedly taken off from Fairford on missions to Iran since the war there began in February. The previous British prime minister, Keir Starmer, initially refused to allow the Trump administration to use the base for strikes at the start of the conflict, but later gave permission for what he called “defensive” operations. His successor, Prime Minister Andy Burnham, has continued that policy, officials said.

Richard Ocone, the assistant chief constable for the Gloucestershire Police, the local force in the area containing the air base, said the incident had begun around 12:45 a.m., when his department received a call about three suspicious vehicles traveling toward the base.

“The emergency services have well-rehearsed plans for dealing with major incidents such as this,” he said, adding that “we are working closely with our partners in counterterrorism policing.”

Earlier, a police statement said that the authorities believed the incident was “contained.”

Prime Minister Burnham said in a statement Sunday afternoon that he understood the incident would be “deeply concerning for local residents and for many people following developments across the country.” He thanked the police and the armed forces, and said, “The investigation remains ongoing, and I would urge people not to speculate while the authorities establish the full facts.”

The United States Air Force referred questions about the incident at the base, which is home to both the 501st Combat Support Wing Headquarters and the 420th Air Base Squadron, to the British authorities.

In a statement, it said: “U.S. European Command remains vigilant and will take appropriate actions to ensure the safety and security of our U.S. service members, civilians, contractors, and their families across the European theater. We constantly assess a variety of factors that determine what measures we institute or modify to protect our installations, our people and their families.”

Fairford has been an American base of operations for decades. The station opened in preparation for D-Day in 1944. With a runway more than 9,990 feet long, it is one of only a few European airfields that can be used by heavy U.S. bombers.

In the past two years, there have been several so-called hybrid attacks — hostile actions devised to evade culpability — in Britain and Europe that intelligence officials say were probably carried out by Russia or people connected to it, or by proxies operating on behalf of Iran.

In Britain, two men were convicted in June of conspiring to carry out arson attacks targeting properties and a car linked to Mr. Starmer after one of them was recruited by a Russian-language Telegram account. The fires at two houses and a vehicle, all of which Mr. Starmer had owned or used, happened over five days in May 2025.

And in July, the British government accused a proxy of Iran’s Revolutionary Guards of directing a wave of actions against Jews in Britain and Europe, including a series of arson attacks on synagogues.

Adam Goldman contributed reporting from London