Britain said on Tuesday that its military bases were safe, after the US removed huge refuelling aircraft from a British airbase following what the Wall Street Journal said was a threat from Iran, the second such apparent evacuation in three months.

Commercially available flight tracking data showed at least 10 Boeing KC-135R Stratotanker refuelling planes left the Royal Air Force base in Mildenhall, eastern England, on Monday. One flew to a base in Germany, one to Scotland and two to Spain, while the destinations of the other six were not reported.

At least three Osprey multipurpose aircraft also departed from Mildenhall and landed at Lakenheath, another RAF base 4 miles away.

Video filmed by a member of the public from outside the base and verified by Reuters showed two Ospreys taxiing on the runway after landing at Lakenheath.

Asked about the moves, the US European Command said the Air Force moves aircraft between locations for a variety of reasons, and it would not comment on force protection measures for operational security reasons.

A spokesman for Prime Minister Andy Burnham said the protection of Britain’s military bases was a top priority, and that the government worked closely with the police and allies to protect them.

Security measures were kept under constant review and the government was investing in air defence and counter-drone capabilities, he said.

Asked if he was confident that Britain’s bases were safe, he said: “Yes.”

The White House and the Pentagon did not immediately respond to requests for comment sent outside regular business hours.

US removed bombers in September

According to the Wall Street Journal, the evacuations were prompted by a threat from Iran. The precise nature of the threat was under discussion among US officials, it said.

Iran has denied any links to specific attacks in Britain, but its Islamic Revolutionary Guards Corps (IRGC) have said airbases used by US forces there and in other countries are legitimate targets. Britain has been warning for months that Iran has been planning attacks, recruiting people online to carry them out.

In September, the US removed bombers that had been used to strike Iran from another British base, RAF Fairford, on security grounds after a group of men approached the base in white vans.

Two men were charged last month in relation to that incident, with prosecutors alleging one of them had tried to assist Iranian intelligence in Britain.

Britain’s domestic spy chief Ken McCallum said in a speech last month that Iran-linked operations had led to more than 50 arrests over the past year.

Iran was ramping up attempts to hit harder targets such as military sites in the UK, even though its previous activity had been limited by the lack of sophistication of the criminals it used as proxies, he said.

The US upgraded the security posture at RAF Mildenhall and another UK base, RAF Lakenheath, to FPCON Charlie, the second-highest defensive threat level, the Wall Street Journal report said, citing a US official familiar with the matter.

The Fairford incident, which remains largely unexplained, attracted huge international attention at a time of heightened concern about the security implications for Western states of the war between the United States and Iran.

Burnham has said the activity at RAF Fairford was “highly likely” to have been coordinated in Iran.