Britain insisted Tuesday that its military bases remained safe after the United States removed at least 10 large refueling aircraft from a British airfield. The movement followed what the Wall Street Journal reported was a threat from Iran.

Commercial flight tracking data showed at least 10 Boeing KC-135R Stratotanker aircraft leaving the Royal Air Force base at Mildenhall in eastern England on Monday. It was the second apparent evacuation involving American military aircraft in three months.

One Stratotanker flew to a base in Germany, another traveled to Scotland, and two went to Spain. The destinations of the other six aircraft were not reported in the available tracking data.

At least three Osprey multipurpose aircraft also departed Mildenhall and landed at RAF Lakenheath, about 4 miles away. Video recorded by a member of the public and verified by Reuters showed two Ospreys taxiing after landing at Lakenheath.

The United States also removed its B-1 bombers from a British air base one week after several men were arrested there on suspicion of preparing terrorist acts. American officials offered little public detail about the aircraft movements.

U.S. European Command said the Air Force routinely moves aircraft between locations for a variety of reasons. The command declined to discuss force protection measures, citing operational security.

A spokesman for Prime Minister Andy Burnham said protecting Britain’s military bases was a top priority. He said the government was working closely with police and allied countries to secure the facilities.

Security measures remained under constant review, according to the spokesman. Britain was also investing more than £2 billion ($2.7 billion) in air defense and counter drone capabilities, and when asked whether the bases were safe, he answered: “Yes.”

The White House and the Pentagon did not immediately respond to requests for comment sent outside regular business hours. The Wall Street Journal reported that the aircraft evacuations were prompted by an Iranian threat, although the precise nature of it remained under discussion among U.S. officials.

Iran has denied links to specific attacks in Britain. Its Revolutionary Guards, however, have said airbases used by American forces in Britain and other countries are legitimate targets.

British authorities have warned for months that Iran has been planning attacks and recruiting people online to conduct them. Those warnings have added urgency to security concerns surrounding military installations hosting American forces.

In September, the United States removed bombers used to strike Iran from RAF Fairford on security grounds. That removal came after a group of men approached the base in white vans.

Two men were charged last month in connection with the Fairford incident. Prosecutors alleged that one of them had tried to assist Iranian intelligence in Britain, while the wider circumstances surrounding the incident remained largely unexplained.

Ken McCallum, Britain’s domestic spy chief, said in a speech last month that operations linked to Iran had resulted in more than 50 arrests during the previous year. He said Iran was increasing attempts to strike harder targets, including British military sites.

McCallum said earlier activity had been constrained by the limited sophistication of criminals used as proxies. Burnham separately said the activity at RAF Fairford was “highly likely” to have been coordinated in Iran.

The Wall Street Journal reported that the United States raised the security posture at RAF Mildenhall and RAF Lakenheath to FPCON Charlie. The designation is the second highest defensive threat level, according to the report, which cited a U.S. official familiar with the matter.

The Fairford incident drew substantial international attention amid heightened concern about the security consequences for Western countries of the war between the United States and Iran. Now, another movement of American aircraft has placed Britain’s assurances about base security under renewed scrutiny.