President Donald Trump has said the US was “cocked and loaded” to retaliate against Iran for downing an American drone, but cancelled the strikes 10 minutes before they were to be carried out after being told some 150 people could die.
Mr Trump tweeted on Friday that the US was ready to “retaliate last night on 3 different sights (sic) when I asked, how many will die”.
He said a general told him 150 people, and he cancelled the strikes as “not proportionate to shooting down an unmanned drone”.
Mr Trump tweeted that the US will never allow Iran to have a nuclear weapon.
But he said he was in no hurry to respond to the downing of the US surveillance drone over the Strait of Hormuz.
He said US sanctions are crippling the Iranian economy and that more are being added.
The targets on Thursday night would have included radars and missile batteries, the official said earlier.
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 have mounted in recent weeks, there have been growing fears that either side could make a dire miscalculation that leads to war.
According to the official, the strikes were recommended by the Pentagon and were among the options presented to senior administration officials.
It was unclear how far the preparations had gone but no shots were fired or missiles launched, the official said.
The military operation was called off at around 7.30pm Washington time, after Mr Trump had spent most of Thursday discussing Iran strategy with top national security advisers and congressional leaders.
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.
Mr 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,” Mr Trump said at the White House.
“I think that it could have been somebody who was loose and stupid that did it.”
Mr Trump, who has said he wants to avoid war and negotiate with Iran over its nuclear ambitions, cast the shootdown 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.
But fears of open conflict shadowed much of the discourse in Washington.
Mr Trump summoned his top national security advisers and congressional leaders to the White House for an hour-long briefing in the Situation Room.
Those attending included secretary of state Mike Pompeo, national security adviser John Bolton, CIA director Gina Haspel, joint chiefs chairman General Joseph Dunford, acting defence secretary Patrick Shanahan and army secretary Mark Esper, whom Mr Trump has said he will nominate as Pentagon chief.
Mr Pompeo and Mr Bolton have advocated hardline policies against Iran, but Representative Adam Schiff, the chairman of the House intelligence committee, said “the president certainly was listening” when congressional leaders at the meeting urged him to be cautious and not escalate the already tense situation.
On Capitol Hill, leaders urged caution and some politicians insisted the White House must consult with Congress before taking any actions.
House Speaker Nancy Pelosi said no specific options for a US response were presented at the meeting.
Senate Majority Leader Mitch McConnell said: “The administration is engaged in what I would call measured responses.”
The Trump administration has been putting increasing economic pressure on Iran for more than a year.
It reinstated punishing sanctions following Mr Trump’s decision to pull the US out of an international agreement intended to limit Iran’s nuclear programme in exchange for relief from earlier sanctions.
The other world powers who remain signed on to the nuclear deal have set a meeting to discuss the US withdrawal and Iran has announced plans to increase its uranium stockpile for June 28, a date far enough in the future to perhaps allow tensions to cool.
On Thursday, Iran called the sanctions “economic terrorism”.
Citing Iranian threats, the US recently sent an aircraft carrier to the Persian Gulf region and deployed additional troops alongside the tens of thousands already there.
All this has raised fears that a miscalculation or further rise in tensions could push the US and Iran into an open conflict 40 years after Tehran’s Islamic Revolution.
“We do not have any intention for war with any country, but we are fully ready for war,” Revolutionary Guard commander General Hossein Salami said in a televised address.
The paramilitary guard, which answers only to Supreme Leader Ayatollah Ali Khamenei, said it shot down the drone at 4.05am on Thursday when it entered Iranian airspace near the Kouhmobarak district in southern Iran’s Hormozgan province.
Kouhmobarak is about 1,200 kilometres (750 miles) south east of Tehran.
Taking issue with the US version of where the attack occurred, Iranian foreign minister Mohammad Javad Zarif tweeted that his country had retrieved sections of the military drone “in our territorial waters where it was shot down”.
He said: “We don’t seek war but will zealously defend our skies, land & waters.”
Air force Lt Gen Joseph Guastella, commander of US Central Command air forces in the region, disputed that contention, telling reporters that the aircraft was 21 miles from the nearest Iranian territory and flying at high altitude when struck by a surface-to-air missile.
The US military has not commented on the mission of the remotely piloted aircraft that can fly higher than 10 miles in altitude and stay in the air for more than 24 hours at a time.
“This attack is an attempt to disrupt our ability to monitor the area following recent threats to international shipping and free flow of commerce,” he said.
Late on Thursday, the Federal Aviation Administration barred American-registered aircraft from flying over parts of the Persian Gulf and the Gulf of Oman.
Democratic leaders in particular urged the president to work with US allies and stressed the need for caution to avoid any unintended escalation.
Senator Chuck Schumer said he told Mr Trump that conflicts have a way of escalating and “we’re worried that he and the administration may bumble into a war”.