(BSS/AFP) - A UK probe into a suspected bomb plot at an airbase used by the United States was mired in confusion Tuesday, as police said no explosives but a "quantity of petrol" was found in vehicles seized nearby.

Police were called to the Royal Air Force Fairford base in southwest England on Sunday after a resident saw several vans "acting suspiciously", triggering an evacuation of the nearby village and the declaration of a "major incident".

"Those vehicles have been subject to extensive examination... and we can confirm that no improvised explosive devices were found. However, a quantity of petrol was recovered," National Counter Terrorism Policing head Laurence Taylor said.

The development came as British media reported that a member of the group called police around an hour before a local farmer alerted officers after spotting the men and vans.

There have been huge questions around the incident after five London-based Britons were released on bail Monday after being arrested early Sunday on explosives and terror offences, with Washington criticising their release.

"I understand why this development will have come as a surprise to many, but I`d like to reiterate that it was an investigative decision," Taylor added, after US President Donald Trump said he was "surprised" by the decision.

"I wouldn`t have done that," Trump told reporters, as his Secretary of State Marco Rubio alleged a "foreign actor" was involved.

Speculation was rife about the incident near the Fairford airbase, which is used by the US military to launch strikes on Iran.

Iran`s embassy in London has said it "categorically rejects and strongly condemns" speculation that Tehran was involved.

The latest update lends credibility to some theories that the men could be fuel thieves, as energy prices rise in the UK.

But the scare forced 85 homes in a nearby village to be evacuated, with many residents having to stay elsewhere for at least one night.

Police said they had searched homes in London and were hoping to stand down investigative operations on Wednesday.

- `Embarrassing` -

A resident in the neighbouring village of Kempsford, Patricia, 60, told AFP it was "very embarrassing that people around the world thought it was a terrorist attack. But in the end, no -- it`s a lot of noise for nothing".

The security scare became public after a farmer living in the village of Whelford near the airbase rang police after seeing the three vans blocking a road and a group of masked men fleeing the scene.

British media later reported that officers had already been alerted by one of the suspects before the farmer`s call, although police have not publicly commented on the reports.

Armed officers arrested the five men -- all London residents in their mid-20s -- on suspicion of explosives offences and, later, "suspicion of preparing a terrorist act" under anti-terror laws.

Police said the men "remain under investigation and must adhere to strict conditions while we continue our work".

Security at the base had reportedly been heightened recently.

Britain gave Washington permission in March to use Fairford for "specific defensive operations into Iran" to destroy Iranian missiles at source.

US bombers and other planes have since been seen taking off and landing from the base -- the only one on Britain`s mainland used for this purpose.

Aerial footage of the vans showed they appeared to contain barrels, and at least one was marked with a fake logo and phone number, giving the false impression that it was owned by a legitimate diesel delivery firm.

The legitimate firm said in a statement on its website that "neither the vehicle nor any of the individuals involved has any connection whatsoever with Fuel2U", adding it was cooperating with the police.

"It`s a bit odd they were here in the middle of the night," one local told AFP as she did some gardening in the nearby village of Kempsford.

"If they were true terrorists, they wouldn`t have released them," she added, asking not to give her name.

A British military camp was the victim of a "security breach" last year after intruders cut through a fence to steal fuel from a tanker.