LONDON — The United States has moved military aircraft from an airbase in the United Kingdom amid threats from Iran.

A U.S. official confirmed to NBC News that several military aircraft aircraft had left RAF Mildenhall as the security posture there had increased.

They were operating under “enhanced force protection measures,” the official said. They would not speculate on the exact nature of the threat that appears to have led to ramped up security.

At least eight air refueling tankers and three Ospreys left the base in Suffolk, eastern England, on Monday, flight tracking data showed.

Some of the tankers, Boeing KC-135 Stratotankers, were relocated to airbases in Germany and Spain, with one moved to the area of Glasgow Prestwick Airport in Scotland. The Ospreys were moved to nearby RAF Lakenheath in Suffolk.

It comes weeks after the U.S. withdrew bombers from another base in Britain, RAF Fairford, after police foiled a suspected terror plot linked to Iran. The U.S. has used bases in the U.K. to strike the Islamic Republic.

The Wall Street Journal was first to report on this week’s movements and said the evacuation came following a threat from Iran, citing people familiar with the matter. It said the exact nature of the threat was not yet known and remained under discussion by U.S. officials.

“We take proactive measures to ensure our aircraft, our people, and our installations remain safe and secure,” a U.S. official told NBC News.

“U.S. Air Force aircraft rotate between installations and operating locations for a variety of reasons including training and real world requirement,” a U.S. European Command official said. “U.S. military aircraft are hosted in accordance with access, basing, and overflight agreements with our Allies and partners. Due to operations security, we will not provide further information on movement of assets,” the European Command official added.

A spokesperson for the U.K.’s Ministry of Defense said it does “not routinely comment on the activity of other nations.”

“Aircraft and other assets move regularly according to operational requirements,” the spokesperson said.

It comes weeks after the U.S. pulled aircraft from RAF Fairford following reports of a possible Iranian drone attack, two U.S. officials previously told NBC News. That move came in the wake of an investigation into a possible planned terrorist attack at that base, located in Gloucestershire.

Five men were detained on Sept. 27 after three suspicious vans were spotted traveling toward RAF Fairford. Other arrests followed and two men were charged under the National Security Act.

The incident, which attracted huge international attention, remains largely unexplained although British Prime Minister Andy Burnham said the activity was likely to have been coordinated in Iran.

Though Tehran has denied involvement in any plot against RAF Fairford, its powerful Revolutionary Guard warned in July that any base used to launch attacks against Iranian territory was a legitimate target.

Iran has vowed retaliation for any renewal of the American-Israeli war against it and has threatened to expand its response beyond the Middle East.

Secretary of State Marco Rubio said Monday that Washington had addressed concerns around reported threats from Tehran against American diplomats overseas by “moving people away from the threat.”

“Obviously, we’re always worried about the safety and security of our personnel and their families overseas,” Rubio said. “And so, when we see anything that could pose a potential threat beyond a certain threshold, we’re going to address it.”

Citing new “momentum” in negotiations to end the war, President Donald Trump on Monday downplayed the prospect of new U.S. attacks.

“Iran militarily is effectively over,” Trump said following a meeting with Qatar’s Prime Minister Sheikh Mohammed bin Abdulrahman Al-Thani.

Further U.S. attacks “won’t be necessary” at this point, but Washington remains “locked and loaded” should circumstances change, Trump told reporters at the White House.