The US military is moving planes from RAF Mildenhall in the UK to Germany and Spain, apparently due to the Iranian threat.

The U.S. military on Monday moved nine Stratotanker refueling planes from a base in England to facilities in Germany and Spain, and the Wall Street Journal cited sources saying they were repositioned because of an Iranian threat.

It is unclear whether the threat was imminent or general, the sources told the newspaper.

CBS News Confirmed found public aircraft tracking data showing 10 KC-135 Stratotankers were moved from Royal Air Force Base Mildenhall in southeast England to other locations. One landed at Glasgow Prestwick Airport in Scotland, while four flew to Spangdahlem Air Base in Germany and one landed at Ramstein Air Base, the headquarters of U.S. Forces Europe, also in Germany.

Four more Stratotankers left Mildenhall and arrived at Morón Air Base in southern Spain on Monday.

“We are taking proactive steps to ensure the safety of our aircraft, our employees and our facilities,” a U.S. official told CBS News on Tuesday, without commenting on specific aircraft movements.

Three US Osprey tilt-rotor transport planes also left RAF Mildenhall and arrived on Tuesday at RAF Lakenheath, another base used by the US in England. It is unclear whether all of these movements were linked to the reported Iranian threat.

A U.S. Air Force spokesperson told CBS News that the aircraft “regularly rotate between installations and operating sites for a variety of reasons, including training and real-world requirements.”

The spokesperson added that “US military aircraft are hosted in accordance with access, basing and overflight agreements with our allies and partners. Due to operational security, we will not provide additional information on asset movements.”

The U.S. Air Force’s 100th Air Refueling Wing, based at RAF Mildenhall for years, did not immediately respond to CBS News’ request for comment on the planes’ movements. The 100th Air Squadron is the only permanent U.S. refueling wing in Europe.

The relocation of the plane comes a little more than a month after that of the US Air Force decided to delete all its long-range bombers from RAF Fairford, another air base in England which had been used to launch strikes against Iran, following a security incident.

On September 27, five Britons were arrested for suspected explosives and terrorist offenses near the Fairford base. The suspects were later released on bail. A month later, two men were charged with security violations in connection with the incident.

Mohmed Abdulkarim, 24, was charged with “assisting a foreign intelligence service” and “engaging in conduct likely to materially assist a foreign intelligence service in carrying out activities relating to the United Kingdom”.

Nouman Essghir, also aged 24, was charged with a national security offense after he “approached or was in the vicinity of a prohibited place and his conduct was for a purpose which he knew, or in view of other facts of which he was aware, which he reasonably ought to have known, was detrimental to the security or interests of the United Kingdom”.

British Prime Minister Andy Burnham said at the time that Iran may have “played a role” in the September incident, although he did not provide details.

“There are strong indications that Iran played a role in what happened this weekend at RAF Fairford, but of course this is an ongoing, complex and serious police investigation, and of course we are working closely with our colleagues in the United States of America,” Burnham told the BBC in October.

British counter-terrorism police said no explosives were found during extensive searches of the three vehicles driven by the men arrested near RAF Fairford, only fuel and a number of large, mostly empty plastic drums.