LONDON, U.K.: The United States has moved several military aircraft from a second airbase in Britain following a security threat linked to Iran, raising concerns about the safety of U.S. military facilities in the country.

Two U.S. refuelling aircraft operating from RAF Mildenhall were diverted to Germany early this week. Several other tankers based there were moved to Germany and other locations later in the day, according to a report by The Wall Street Journal, which cited people familiar with the matter.

U.S. officials are still discussing the exact nature of the threat against RAF Mildenhall.

The U.S. military also raised the security alert level at RAF Mildenhall and RAF Lakenheath to FPCON Charlie, the newspaper reported, citing a U.S. official familiar with the situation.

FPCON Charlie is the second-highest security alert level used by the U.S. Department of Defense. It is introduced when intelligence suggests a terrorist attack or similar action is likely.

A British Ministry of Defense spokesperson said, "We do not routinely comment on the activity of other nations. Aircraft and other assets move regularly according to operational requirements."

A U.S. Air Force spokesperson did not provide details about the movement of military aircraft to the Wall Street Journal. However, the spokesperson said U.S. aircraft regularly move between military bases and operating locations for different operational reasons.

Reuters said it could not immediately reach the U.S. Air Force for comment. The White House, Pentagon and U.S. European Command also did not immediately respond to requests for comment sent outside normal working hours.

The latest development comes weeks after the U.S. removed its bombers from RAF Fairford in southwest England following a suspected security incident linked to Iran.

Five men were arrested outside the airbase in September in connection with the incident. Two other men were later arrested on suspicion of helping Iranian intelligence.

The circumstances remain largely unclear. However, it attracted international attention because of growing concerns about the security risks facing Western countries during the war between the United States and Iran.

After the incident, Washington moved its B-1 bombers away from RAF Fairford on security grounds. U.S. bombers had used the base to launch attacks on Iran.

British Prime Minister Andy Burnham said last month that the activity around RAF Fairford may have been coordinated in Iran.

Iran has denied involvement in any plot against the British airbase. Tehran's ambassador to Britain rejected the allegations.

However, Iran's Revolutionary Guards had warned in July that RAF Fairford and any other military base used to launch attacks against Iran would be considered legitimate targets.