The U.S. moved refueling tankers based at RAF Mildenhall in the United Kingdom to a base in Germany because of an unspecified Iran threat, according to a source familiar with the matter.

The force protection levels at RAF Mildenhall and RAF Lakenheath were increased to FPCON Charlie, according to the source.

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