The U.S. Air Force has relocated aircraft from RAF Mildenhall in eastern England because of a threat, a U.S. official told the Associated Press — the second time in under two months that American aircraft have been moved off a British base over a suspected Iranian threat.

The Wall Street Journal, which first reported the evacuation, said the threat related to Iran, citing people familiar with the matter. The Military Air Tracking Alliance, a team of about 30 open-source analysts that tracks military flight activity, said it spotted 13 U.S. aircraft leaving Mildenhall for bases in Scotland, Spain and Germany. Two tankers were diverted to Germany on Monday morning, and Sky News counted 12 aircraft leaving on Monday — nine tankers and three Osprey transporters — while CNN's analysis of flight-tracking data showed 10 refueling aircraft relocated.

The base also raised its alert level to FPCON Charlie, the second-highest level, which applies when intelligence indicates that some form of terrorist action targeting personnel or facilities is likely. The security posture was also upgraded at RAF Lakenheath, another U.S.-operated base in England.

A U.S. official described the move as proactive rather than reactive, adding that some of the planes were in transit while others were permanently stationed at Mildenhall, and that they may return. The Pentagon gave no immediate confirmation. An official from the U.S. Air Force in Europe told the AP that the U.S. is "taking coordinated and appropriate actions" to ensure the safety and security of U.S. and U.K. personnel and their families. Secretary of State Marco Rubio said the United States is "constantly repositioning forces around the world for a variety of reasons" and referred questions to the Pentagon.

The move came less than two months after the Air Force withdrew its B-1 bombers from RAF Fairford, another U.S.-run base in Britain, as police investigated a suspected Iran-linked terror plot. Five men were detained on September 27 after three suspicious vans were spotted travelling toward Fairford, and two men have since been charged under the National Security Act. Bombers flying out of RAF Fairford had been used to strike Iran during the Middle East conflict.

The director of national intelligence, Jay Clayton, said in October that Iran was in "advanced stages" of planning to target U.S. personnel in the Middle East, and Tehran has said its retaliation for any U.S. attack on Iran would expand beyond the Middle East. Mildenhall hosts the 100th Air Refueling Wing, the only permanent refueling wing in the European region, operating the KC-135 Stratotanker — a four-engine jet with a wingspan of 131 feet that can carry up to 83,000 pounds of cargo.