The United States military evacuated aircraft from a second airbase in Britain on Monday after a threat from Iran, the Wall Street Journal reported, citing people familiar with the matter, weeks after Washington withdrew its bombers from RAF Fairford airbase over a suspected Iran-linked security incident. Two US refueling tankers that were using the RAF Mildenhall military base diverted to Germany earlier on Monday, while several other tankers at the base were moved to Germany and other locations later in the day, the WSJ report said. According to the Journal, the precise nature of the threat to RAF Mildenhall remains under discussion among US officials. The US upgraded the security posture at RAF Mildenhall and RAF Lakenheath to FPCON Charlie, the WSJ report said, citing a US official familiar with the matter. Force Protection Condition (FPCON) Charlie is the second-highest defensive threat level in the US Department of Defence's security system and occurs when intelligence indicates that a form of terrorist action is likely. Read: Trump says men arrested at US air base in UK were looking to do 'big damage' “We do not routinely comment on the activity of other nations. Aircraft and other assets move regularly according to operational requirements,” a British ministry of defence spokesperson said. A US Air Force spokesperson declined to provide to the Journal details about the movement of assets, but said US aircraft rotate between installations and operating locations for a variety of reasons. Reuters could not immediately reach the US Air Force, while the White House, the Pentagon and the US European Command did not immediately respond to requests for comment sent outside regular business hours. Five men were arrested outside RAF Fairford airbase in southwest England in September over a security incident with two others arrested later on suspicion of assisting Iranian intelligence. Read More: Iran warns US of 'devastating response' if Washington starts war The circumstances of the incident, which remain largely unexplained, attracted huge international attention, coming at a time of heightened concern about the security implications for Western states of the war between the US and Iran, and prompted the US to remove its B-1 bombers from the Fairford airbase on security grounds. RAF Fairford had been used by US bombers to carry out strikes on Iran. British Prime Minister Andy Burnham said last month that the activity at RAF Fairford was likely to have been coordinated in Iran. While Tehran's ambassador to Britain has denied accusations that it was involved in any plot against the airbase, Iran's Revolutionary Guards said in July that the Fairford base and any other base used to launch attacks against Iran were legitimate targets.