The United States has withdrawn all B-1 bombers from RAF Fairford in Britain due to reported "new threats," though the British government insisted Monday the site is safe.

Police were called to the base in southwest England on Sept. 27 after a local farmer reported a group of men with several vans engaged in "suspicious" activity.

The scare prompted a counterterrorism probe and forced evacuations at 85 homes near the airbase, which was used by the United States to launch strikes on Iran.

Investigators did not find any explosives in the vans, but "a quantity of petrol was recovered," according to counterterrorism police.

A statement from a Pentagon official Sunday said "we can acknowledge now that all U.S. bombers that were deployed to RAF Fairford have re-deployed to their home stations in the United States."

The statement noted that "operational security precluded us from confirming the movement of our assets and forces in real-time."

An unidentified U.S. official told CNN that the bombers were moved after "an unspecified new threat," with the source calling the latest threat a "real world incident."

It was unusual to move the bombers back to their home stations instead of deploying them to another base in the region, the official reportedly said.

Two U.S. officials told the New York Times that the bombers were moved after commanders received new threats about an Iran-backed plot against the base.

One American official said the new information was part of the same threat warnings that surfaced three weeks ago, but the situation became urgent in the past 24 hours, the Times reported.

Asked whether the base was safe, U.K. Prime Minister Andy Burnham's official spokesman said: "Yes."

"We have tried and tested approaches to defend our bases around the country and throughout the world," he told reporters.

Heightened security

Speculation about the motives behind the incident ranged from terror threat to fuel theft, as energy prices in the U.K. hit record highs this week.

Burnham has said there were "strong indications that Iran played a part" in the incident, a charge rejected by Tehran.

Six London-based Britons arrested in the September incident, including a U.K.-Iranian dual national, all men in their mid-20s, were released on bail, drawing rebuke from Washington.

"I wouldn't have done that," U.S. President Donald Trump told reporters, as his Secretary of State Marco Rubio alleged a "foreign actor" was involved.

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

Adding to the mystery, British media reported that officers were alerted to the incident by one of the initial suspects, but police have not commented on the reports.

Security at the base had reportedly been heightened recently.

Britain gave Washington permission in March to use Fairford for "specific defensive operations into Iran," and U.S. bombers and other planes have since been seen taking off and landing at the base.

Fairford is the only airbase on Britain's mainland used for this purpose.

The incident came after a farmer living in the village of Whelford, near the airbase, called the police after seeing three vans blocking a road and a group of masked men fleeing the scene.

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.