The United States rushed to withdraw 12 B-1 bombers from an air base in southern England over the weekend. Officials said the move followed new threats about an Iran-backed plot. Some planes left while refueling aircraft were still being scrambled. By Sunday, at least three bombers reached South Dakota safely.

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.

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