The United States rushed over the weekend to withdraw all 12 B-1 Air Force bombers from an air base in southern England after commanders received new threats about an Iran-backed plot against the installation, two U.S. officials said on Sunday.
The withdrawal was highly unusual: Such moves are rarely made so quickly from an allied air base in a peacetime country thousands of miles from a combat zone.
The need to move the bombers out of the base, R.A.F. Fairford, was deemed so urgent that some of the planes and their flight crews left while aerial refueling aircraft were still being scrambled to support their flights to the United States. At least three of the planes had landed at their home base in South Dakota by Sunday, the officials said, speaking on the condition of anonymity to discuss operational matters.
The Pentagon’s European Command confirmed on Sunday that the bombers had left England but declined to comment on any new warnings.
“We can acknowledge now that all U.S. bombers that were deployed to R.A.F. Fairford are redeploying to their home stations in the United States,” the command said in a statement. “U.S. Air Force bombers routinely operate around the world to support global force readiness and operational requirements.”
The British Ministry of Defense declined to comment on the movements of U.S. military assets.
American officials on Sunday declined to specify the nature of the new information that prompted the rapid withdrawal of the Air Force bombers, some of which had just arrived in the past week as part of a normal rotation of forces. One U.S. official said the new information was part of the same threat warnings that surfaced three weeks ago, but specific details and a sense of urgency spiked in the past 24 hours, alarming commanders.
The sudden pullout of the bombers came a week after the British police arrested five men who had been traveling last Sunday morning in white vans near the base, which has been used by American bombers in the U.S.-Israeli war against Iran. The men were released on bail a day later after the authorities did not discover explosives in the vans.
But the police have repeatedly said the men remain under investigation, and a dual British-Iranian national was arrested on Thursday on suspicion of preparing terrorist acts.
Prime Minister Andy Burnham said last week that the British government believed Iran played a part in planning those events. U.S. and British officials, as well as those in Germany and Israel, believe the operation is connected to Iran’s Islamic Revolutionary Guards Corps or a separate military headquarters in Tehran. Iran has denied the accusations.
The episode has presented the authorities with myriad puzzles as they try to sort out the events over the past week, including the arrests, the frantic evacuation of 85 homes, a bomb disposal robot searching the vans for explosives and a national security meeting convened by Mr. Burnham.
In July, Iran’s Revolutionary Guards issued a statement warning that allowing the United States to use the base “makes London responsible” for U.S. actions. “Any base used to launch attacks on Iranian territory,” the statement said, “is a legitimate target for our forces.”
In September, U.S. military commanders received a chilling warning from American and other Western intelligence agencies, according to U.S. officials briefed on the information. R.A.F. Fairford was the target of a potential Iran-backed attack.
The murky outlines of a plot had been pieced together through a combination of intercepted communications, informants and other intelligence. Israel’s prime minister, Benjamin Netanyahu, said his intelligence officials also warned their British counterparts of a probable Iranian-sponsored attack, though they did not provide a specific target. Follow-up attacks were likely to include drones, the confidential assessments warned.
After that initial warning last month, concern among officials across Europe surged. According to a senior U.S. official, Israel warned Germany that American bases there were at heightened risk, noting Spangdahlem and Ramstein in particular.
An Iran-backed plot to commit an attack in Britain would be a major escalation by Tehran against a Western country in apparent retaliation for the American-and Israeli-led war against Iran.
Michael D. Shear contributed reporting from London.
