London, UK - Iran's Revolutionary Guards "almost certainly directed" a major scare at a UK airbase used by the US military for its war in Iran, Britain's Prime Minister Andy Burnham said Monday.

The US abruptly withdrew all its B-1 bombers from Royal Air Force Fairford last week after the mysterious incident at the base on September 27.

Burnham told Parliament that Britain's assessment of the incident was that it "is highly likely to have been coordinated in Iran, almost certainly directed by Iran's Islamic Revolutionary Guard."

"We will not tolerate these threats on our soil. We will crack down on Iran's use of criminal networks, using our full range of powers to target them and their funding streams," he warned.

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The Foreign Office confirmed in a statement Monday that Iran's ambassador to the UK had been summoned for talks.

"We are also announcing the designation of two Iranian nationals for their role in facilitating or engaging in hostile activity on behalf of the Iranian regime, with the clear intent of threatening, planning or conducting attacks overseas," foreign minister Ed Miliband said.

Speculation has swirled about the incident since police were called to the base in southwest England on September 27 after a local farmer reported a group of men with several white vans engaged in "suspicious" activity.

Despite a major counter-terrorism probe, seven suspects arrested in the days following, including a British-Iranian national, have all since been released on bail. None has been charged.

The incident was declared "major" by British authorities and triggered the evacuation of a nearby village.

But the source and nature of the threat have remained shrouded in mystery.

Investigators did not find any explosives in the vans, but did recover some petrol, according to officials.

Miliband said the UK had already "designated the Islamic Revolutionary Guard Corps (IRGC) under our new state threats powers, and imposed major sanctions on those responsible for Iran's malign activity".