President Donald Trump issued an ultimatum Wednesday for Iran to accept a deal to end the war or face intense renewed US bombing, the latest in a series of abrupt policy shifts.

"Assuming Iran agrees to give what has been agreed to, which is perhaps a big assumption, the already legendary Epic Fury will be at an end," Trump posted on his Truth Social platform," using the US name for its military campaign against Iran.

"If they don''t agree, the bombing starts, and it will be, sadly, at a much higher level and intensity than it was before."

He later said in an interview with PBS he was optimistic about reaching an agreement with Iran before his scheduled trip to China next week.

"I think it''s got a very good chance of ending, and if it doesn''t end, we have to go back to bombing the hell out of them," Trump told the broadcaster.