US special forces entered Iran on Friday night to rescue a crew member of a downed fighter jet, The Telegraph reports.
Iran shot down a US F-15 over its airspace, the first fighter jet destroyed by enemy fire since the war began.
Two crew members ejected, reports suggested, triggering a race between Iran and the US to find the stranded American airmen.
One was rescued in an operation involving two US military helicopters and low-flying refuelling aircraft, which were targeted by light weaponry.
Both helicopters were hit by Iranian fire, with one trailing smoke as it returned to Iraqi territory, but they landed safely, officials said.
The whereabouts of the remaining US airman were unknown on Friday night, as Iranian media broadcast images of local militia fanning out in a search operation.
Broadcasts offered a $60,000 bounty for his capture as Iran claimed a propaganda victory in shooting down the US jet.
Reports also surfaced late on Friday of a US A-10 Warthog, a close air support attack plane, crashing near the Strait of Hormuz. US officials said the pilot had been rescued.
Donald Trump declined to say what he would do if the missing airman was harmed.
In an interview with The Independent, he said: “Well, I can’t comment on it because – we hope that’s not going to happen.”