Five men apprehended over an apparent terror plot at a British airbase have been bailed amid reports material found in their vans was “not viable explosives”.

In a surprise twist on Tuesday (AEST), British police said the men, all UK citizens in their early 20s who had been extensively questioned, would be released from custody.

They were detained on Monday after a local farmer raised the alarm about three white vans driving towards RAF Fairford, a base that houses US bombers for missions against Iran.

Iran’s Revolutionary ‌Guards said in July the base was a legitimate target.

Laurence Taylor, who heads Britain’s counter-terrorism police, said the men, who had been arrested on suspicion of explosives offences and planning a terrorist act, would be released under stringent bail conditions later on Monday (local time).

“To be very clear, this does not mean that the investigation is over,” Taylor said outside London police headquarters.

“They absolutely remain under investigation as we explore multiple lines of inquiry.”

The Press Association reported that the material found was not viable explosives.

The five suspects are a 24-year-old from Camden, north London; three men aged 23, 24 and 25 from Westminster, central London, and one aged 24 from Willesden, north-west London.

The five British men were arrested near the RAF Fairford airbase. Photo: Supplied

US President Donald Trump reacted on Tuesday morning (AEST) to their release.

“I’m surprised they released them, I wouldn’t have,” he said at the White House.

“We know everything about them; we would not have released them.”

A source familiar with the investigation had previously told Reuters that an Iranian-linked motive was considered the most likely, although a Russian sabotage operation or a jihadist plot were also being looked ‌at as possibilities.

Iran’s embassy ‌in London had earlier said it “categorically rejects and strongly condemns the recent unfounded and malicious speculations” that it was involved. It said the accusations would fuel “Iranophobia ​propaganda” in the UK.

Taylor, who did not specifically refer to Iran, said he knew there was significant interest in whether the current geopolitical context had a bearing on the incident.

“Be assured, we are considering this from every possible angle, including that this may be actively committed by proxies or individuals either knowingly or unknowingly working on behalf of a foreign state,” he said.

Taylor said the three white vans had extensively examined, with support from military and forensic experts. Police still had “significant resources deployed at the scene”.

Police at a road block on the road to RAF Fairford. Photo: AAP

On Monday (AEST), Trump said the suspects had been looking to do “big damage” and had been under ⁠investigation by British and American authorities.

However, there were doubts in British media about how much surveillance they were under, given how ​close they got to the base.

The source familiar with the investigation said the arrests were not “intelligence-led”, meaning that they did not know of any plot in advance.

A farmer who ​alerted the police ‌has said she saw a group of “hooded and masked men” running into fields.

The farmer, who has not been identified, said she believed it was her phone call to Britain’s 999 emergency hotline ​that foiled any plot and that she thought she had seen up to 10 people at the scene.

Defence Minister Wes Streeting told the BBC the farmer had only a “partial picture” of the investigation, without providing further clarification.

“As and when we can, we will report back to the public about these individuals and about the nature of the threat,” Streeting told Times Radio.

Prime Minister Andy ​Burnham said he would chair a meeting of the government’s emergency response committee COBRA later on Monday.

After the Fairford arrests, the US embassy issued a security alert saying Americans should “exercise caution” and be “mindful of personal security”.

The British government has authorised the US use of bombers based at Fairford to strike Iranian missile sites attacking ships in the Strait ​of Hormuz.

The US air force said that its personnel at RAF Fairford “remain vigilant” but “for operational security reasons we will not discuss specific force protection measures”.

In Fairford, police imposed a 400-metre cordon around the base and relocated residents from 85 homes in the nearby village of Whelford.

“Thanks to the excellent work of the army’s explosive ordnance disposal ​team, the scene is now safe, and the residents of ⁠Whelford will shortly be returning home,” UK Interior Minister Shabana Mahmood said.

-with AAP, PA and AP