The US evacuated aircraft from Royal Air Force (RAF) Mildenhall air base in the UK on Monday following a threat from Iran, according to people familiar with the matter, The Wall Street Journal reports.

Accordingly, two US tanker aircraft based at Mildenhall air base were sent to Germany. Later, several more tanker aircraft from the same air base were moved to Germany and some other locations.

The precise nature of the threat to RAF Mildenhall—whether imminent or more general—remains under discussion among US officials, the people said.

The decision was made several weeks after an alleged Iranian-related incident at the RAF Fairford air base in the UK. After that incident, US bombers were withdrawn from there.