A major counter-terrorism emergency has unfolded near an airforce base in Britain used by the US to strike Iran, after a tip-off from a local farmer.

British police descended on the RAF Fairford and arrested five men on suspicion of explosives and terrorism offences on Sunday (local time).

The men were seen travelling in three white vans through a nearby village and towards the airbase in the west of England.

Residents alerted authorities to the suspicious activity, with the tip-offs triggering a massive emergency response early on Sunday.

Locals in 85 homes in the ​nearby village were evacuated, and armed officers were patrolling the site.

Police said there was a ⁠400-metre cordon was around the RAF base as the bomb disposal unit investigated the vans.

US President Donald Trump said the suspects had been looking to do “big damage” ‌and had been under investigation by British and US authorities.

A local farmer told the Daily Telegraph newspaper she had alerted police after seeing the vans and men acting suspiciously as she drove home from a party.

“I saw a group of Middle Eastern-looking young guys running off through the trees. Some of them had masks ​or ​balaclavas on, and they all scattered ahead of us as we drove back past,” she said.

“Some of them ran into the fields, and some of them turned to run towards the ‌air base … I mean, you don’t see masked men running around at half one in the morning in the countryside near a secure air base and not have concerns.”

Three white vans were intercepted near the British base that is used by the US. Photo: Reuters screenshot

Five men were arrested by armed officers and later held on suspicion of preparation of a terrorist act.

An Iranian-linked motive is considered the most likely, a source familiar with the investigation said, while a Russian sabotage operation or an Islamist plot were also being looked at.

“The arrest in the UK was fantastic,” ‌Trump said, saying it ​was “an amazing job”.

“We had them under investigation. They were looking to do big damage to our fort and working with the British worked out great … We had them under view for a long time, ​and we got them,” ‌Trump said.

British Counter Terrorism Policing Senior National Coordinator Vicki Evans said the investigation was in its early stages, with five men in custody.

“As you would expect we are working closely with ‌our partners to understand the ‌circumstances of the incident,” she said.

A major incident was declared at RAF Fairford in the west of England. Photo: AAP

The British government, which has allowed the US to use bombers based at Fairford to strike Iranian missile sites that were attacking ships in the Strait of Hormuz, said Prime Minister Andy Burnham was being updated on the incident.

“I know today’s incident near RAF Fairford will be deeply concerning for local residents and for many people following developments across the country,” Burnham posted on X, also thanking police and the armed forces.

Britain said in July its armed forces were ready to protect the country from any attack after Iran’s Revolutionary Guards warned it not to allow US bombers to fly from ⁠Fairford, saying any base used to launch attacks would be a legitimate target.

On Sunday morning (local time) the base was blocked by armed police while large machinery also prevented access. Reuters witnesses said security had been increased at other bases used by the US, with armoured ​vehicles and soldiers at the gates of RAF Lakenheath and RAF Mildenhall in eastern England.

Local councillor Tristan Wilkinson told Reuters there had been a recent security alert at Fairford and surveillance had been stepped up.

“There was a specific threat about six weeks ago from the Iranians on this base. At the time, there was increased surveillance around the area,” he said.

A spokesperson for the US Air Force declined to discuss specific force protection measures at the base.

“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 spokesperson said.

-with AAP