The suspected terror plot against an RAF base used by American forces in Britain “clearly involves the hand of a foreign actor”, US secretary of state has claimed.

British police declared a major incident at the RAF Fairford base around 90 miles west of London after arresting five men accused of suspected explosives and terrorism offences on Sunday.

Five suspects have been released, with UK police saying they were investigating possible state involvement.

“What happened, what almost happened, what could have happened in the UK over the weekend is a very serious situation,” secretary of state Marco Rubio told the Fox News network’s ‘Hannity’ programme late on Monday.

“It’s one that clearly involves the hands of a foreign actor. I won’t get into great detail about that yet,” he continued, without elaborating.

Mr Rubio mentioned Iran during the interview, pointing out it had “openly threatened to attack American interests globally”, but did not directly blame it for the weekend incident in Britain.

He said the events near RAF Fairford, which US forces have used to strike Iran during the ongoing conflict, were a “very serious situation” and thanked the British authorities for their response.

“I have to be careful. There’s some things I cannot say on the air for a lot of different reasons,” he told Fox News.

Mr Rubio mentioned Iran by name during the interview, pointing out it had threatened to attack US interests, but did not directly blame it for the weekend incident.

“We’re going to always take that very seriously; there will be repercussions for that if it ever were to happen or even attempted,” he said.

“There’s a lot more news that’ll come out on this in the days to come, but there’s only so much I can say right now at this point about it, other than this was a very serious thing and it was responded to appropriately,” he concluded.

President Donald Trump said on Monday that the incident “might be” connected to Iran, and also said that he would not have released the five suspects on bail.

“I will tell you, I would have said ‘Don’t let them have bail’. I’m surprised that they released them. I wouldn’t have done that,” he told reporters at the White House.

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

“The arrest in the UK was fantastic,” he told reporters, 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 added.

Iran has denied involvement in attacks or plots in Britain and elsewhere, and on Monday denied any involvement in the incident at Fairford.

British police released all five suspects on Monday but said their investigation was not over and that they were examining whether individuals acting for a foreign state were involved.

The British nationals were detained on suspicion of explosives offences and terrorism on Sunday after three vans were found near RAF Fairford in Gloucestershire, which has been used by the US military to carry out strikes against Iran.

Residents evacuated from about 85 households were able to return home on Monday, but a cordon remained in place around the vehicles as counter terrorism officers continue to examine the scene.

It is understood that the material found was not viable explosives, but police have said the investigation is continuing.

Earlier, the head of UK Counter Terrorism Policing, Assistant Commissioner Laurence Taylor, acknowledged speculation on whether Iran was behind the incident and said the force was considering “every possible angle”.

In a statement outside New Scotland Yard on Monday, Mr Taylor said he knew there was “significant interest in whether the current geopolitical context has a bearing on this incident”.

He said: “At Counter Terrorism Policing, we are working within this context every single day and, at this stage, we are keeping an open mind in this investigation.

“We have, as you would expect, ensured that our protective security advice and posture remain commensurate to the threat.

“But 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.”

Iran, and other hostile states including Russia, have been known to use criminal proxies to carry out crimes on their behalf on UK soil.

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

The country’s London embassy earlier said it “categorically rejects and strongly condemns the recent unfounded and malicious speculations” of Iranian involvement.

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.

Mr Taylor said the men were released on bail under “stringent conditions” on Monday but “absolutely remain under investigation as we explore multiple lines of inquiry”.

Prime Minister Andy Burnham chaired an emergency Cobra meeting on Monday.

After the meeting, Defence Secretary Wes Streeting said the decisions around bail were operational matters for the police and prosecutors, and the investigation remained ongoing.