US President Donald Trump says he blocked a planned military strike against Iran because it would not have been a proportionate response to Tehran's downing of an unmanned US surveillance drone.
"10 minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone. I am in no hurry, our Military is rebuilt, new, and ready to go, by far the best in the world," Trump said in a series of early morning tweets.
Trump said the US had planned to hit three different sites in response to Iran's downing of its drone, but was told that 150 people would have died.
On Thursday, a US official confirmed a New York Times report which cited anonymous senior administration officials as saying that Trump had approved the strikes, but then called them off.
The official, who was not authorised to discuss the operation publicly and spoke on condition of anonymity, said the targets would have included radars and missile batteries.
Asked earlier on Thursday about a US response to the attack, Trump said: "You'll soon find out."
Iranian officials told Reuters that Tehran had received a message from US President Donald Trump through Oman warning that a US attack on Iran was imminent.
"In his message, Trump said he was against any war with Iran and wanted to talk to Tehran about various issues," one official told Reuters, speaking on condition of anonymity.
"He gave a short period of time to get our response but Iran's immediate response was that it is up to Supreme Leader (Ayatollah Ali) Khamenei to decide about this issue."
The swift reversal was a stark reminder of the serious risk of military conflict between US and Iranian forces as the Trump administration combines a "maximum pressure" campaign of economic sanctions with a build-up of American forces in the region.
As tensions mounted in recent weeks, there have been growing fears that either side could make a dire miscalculation that leads to war.
The downing of the US drone - a huge, unmanned aircraft - over the Strait of Hormuz prompted accusations from the US and Iran about who was the aggressor.
Iran insisted the drone violated Iranian airspace, while Washington said it had been flying over international waters.
Trump's initial comments on the attack were succinct. He declared in a tweet that "Iran made a very big mistake!".
But he also suggested that shooting down the drone - which has a wingspan wider than a Boeing 737 - was a foolish error rather than an intentional escalation, suggesting he may have been looking for some way to avoid a crisis.
"I find it hard to believe it was intentional, if you want to know the truth," Trump said at the White House. "I think that it could have been somebody who was loose and stupid that did it."
Trump, who has said he wants to avoid war and negotiate with Iran over its nuclear ambitions, cast the shoot-down as "a new wrinkle ... a new fly in the ointment." Yet he also said "this country will not stand for it, that I can tell you."
He said the American drone was unarmed and unmanned and "clearly over international waters." It would have "made a big, big difference" if someone had been inside, he said.
As the day wore on, Trump summoned his top national security advisers and congressional leaders to the White House for an hour-long briefing.
On Capitol Hill, leaders urged caution, and some lawmakers insisted the White House must consult with Congress before taking any actions.
Australian Associated Press