The United States moved refuelling aircraft out of RAF Mildenhall in eastern England on Monday and raised the security posture at two US-operated British bases, after what one source described as a credible threat from Iran.

CNN reported that flight tracking data it analysed showed 10 US refuelling aircraft leaving Mildenhall on Monday — four KC-135 Stratotankers to Spangdahlem Air Base in Germany, one to Ramstein, four to Morón Air Base in Spain and one to Glasgow Prestwick airport in Scotland. Al Jazeera’s account is more guarded, describing two refuelling tankers and several other tankers moved to “Germany and other locations.”

Security at RAF Mildenhall and RAF Lakenheath was raised to FPCON Charlie, the second-highest level in the Pentagon’s force protection system, which indicates that some form of terrorist or targeting action is considered likely. The move was first reported by the Wall Street Journal.

What the threat is

A person familiar with the intelligence told CNN that the US intelligence community has in recent weeks picked up a range of “varied, extensive, and specific” plots discussed by Iranians against American targets, running from drone attacks on US bases to assassination plots, with several also aimed at targets inside the United States. Whether Iran has the capability to act on them is unclear.

“The Iranians clearly had specific targets and methods in mind,” the source said. CNN said it had approached the Iranian Embassy in London for comment.

A US official called the aircraft movement a “temporary relocation” and declined to discuss any threat. A spokesperson for Prime Minister Andy Burnham said the UK was in “constant communication with” Washington and that “the protection of our bases, both in the UK and across the world, is a top priority.” The UK Ministry of Defence told Al Jazeera it does “not routinely comment on the activity of other nations.”

A US Air Force spokesperson quoted by the Daily Mirror, which carried an ABC News report, said aircraft “rotate between installations and operating locations for a variety of reasons” and that no further detail would be given on asset movements.

An earlier incident, counted differently

All three filings link the move to an alleged Iranian security threat at RAF Fairford in September, which led to B-1 bombers stationed there being sent back to the United States in October.

The accounts differ on the arrests. The Daily Mirror, citing ABC News, says seven men were initially detained in September and later released, and that two of them were subsequently charged under the UK’s National Security Act, with prosecutors alleging one had likely tried to assist Iranian intelligence inside the UK. Al Jazeera says five men were arrested outside the Fairford base, with two others arrested later on suspicion of assisting Iranian intelligence. The totals agree at seven; the sequence does not.

Not reported

None of the three filings says how long the aircraft will stay away, whether the Lakenheath fighter wing moved any aircraft, or what prompted the intelligence assessment to be judged credible now.