BBC News:- President Donald Trump has said the US military was “cocked and loaded to retaliate” against Iran, but he changed his mind 10 minutes before planned strikes.
Attacks on three sites were planned in response to the shooting down of a US unmanned drone this week.
Mr Trump said he had called off strikes after being told 150 people would die.
He tweeted: “10 minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone.”
The late reversal was first reported by the New York Times on Thursday night. The newspaper said the operation had been “in its early stages” when Mr Trump stood the US military down.
“I am in no hurry,” Mr Trump said on Friday. “Our military is rebuilt, new, and ready to go, by far the best in the world.”
Mr Trump said Iran had downed the drone on Monday, despite an earlier US military statement that the incident happened at 23:35 GMT on Wednesday (04:05 Iran time on Thursday).
On Friday, the president told NBC News that he decided not to give final approval to the planned strikes because of the predicted death toll.
“I didn’t like it. I didn’t think it was proportionate,” he said.
Tehran says the unmanned US aircraft entered Iranian airspace early on Thursday morning. The US maintains it was shot down in international airspace.
Tensions have been escalating between the two countries, with the US recently blaming Iran for attacks on oil tankers operating in the region.