One US crew member has been rescued after Iran shot down a warplane, Israeli and US media reported, as the war looked set to intensify with US President Donald Trump threatening more attacks on civilian infrastructure.
Two US sources told Reuters the plane was a two-seat F-15E and a search was under way.
Iran's Revolutionary Guard Corps said it was combing an area near where the plane came down in southwestern Iran.
The regional governor promised a commendation for anyone who captured or killed the pilot.
Israeli media cited Israeli officials as saying one pilot had been rescued. US broadcaster CBS News reported the same, citing two US officials.
Iranian news agencies said US helicopters were flying low on apparent search missions and carried videos of residents shooting at them.
Iranian state tv announces reward for pilot capture
The Iranian military earlier reported the downed aircraft was an F-35, a single-seater.
The Pentagon and US Central Command did not immediately respond to requests for comment.
The prospect of a US pilot being alive and on the run inside Iran raises the stakes for the United States in a conflict that, according to opinion polls, has struggled to win popular support among Americans.
It also presents a challenge to the US military, which faces the twin goals of trying to save the life of an American behind enemy lines while safeguarding anyone involved in perilous rescue missions.
Iranian officials called on civilians to be on the lookout for survivors and have flooded social media with images that purport to show wreckage from the aircraft.
Iranian state media has released images it said show fragments of a downed US fighter jet in central Iran.
Reuters was not able to confirm the location or date when the photos were taken.
Reuters could not verify the type of aircraft seen in the photos, but William Goodhind, a forensic imagery analyst with Contested Ground, said that the plane's tail fin seen in the photos is consistent with that of a F-15E Strike Eagle.
The red stripe seen on the tail fin of the plane in the photos is consistent with the tail fin of a F-15SE Strike Eagle seen in file photos.
The incident follows threats by Mr Trump to bomb the country back to the "Stone Age," including to attack Iran's energy infrastructure and desalination plants, as he presses Tehran to end the war on US terms.
So far, 13 US military service members have been killed in the conflict and more than 300 have been wounded, according to the US Central Command.
No US troops have been taken prisoner by Iran.
While Mr Trump has repeatedly sought to portray the Iranian military as defeated, Reuters has reported on US intelligence showing Iran retains large amounts of missile and drone capability.
As of last week, the US could only determine with certainty that it has destroyed about one-third of Iran's missile arsenal.
The status of about another third was less clear but bombings likely damaged, destroyed or buried those missiles in underground tunnels and bunkers, according to Reuters sources.
The US and Israeli war with Iran has spread across the Middle East, killing thousands of people and hitting the global economy with soaring energy prices that are fuelling fears of inflation in countries around the world.
