President Donald Trump approved retaliatory military strikes against Iran on Thursday before changing his mind, US media report.
The New York Times, citing senior White House officials, says strikes were planned against a "handful" of targets.
They say the operation was allegedly under way "in its early stages" when Mr Trump stood the US military down. The White House has so far made no comment.
This comes after Iran shot down a US spy drone.
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. Iran has announced it will soon exceed international agreed limits on its nuclear programme.
Last year, the US unilaterally pulled out of a 2015 nuclear deal aimed at curbing Iran's nuclear activities.
What do the US media reports say?
The New York Times first published details of the apparent planned strikes late on Thursday night in Washington.
As late as 19:00 local time (23:00 GMT), it said, US military and diplomatic officials still expected the strikes on agreed targets, including Iranian radar and missile batteries, to take place.
"Planes were in the air and ships were in position, but no missiles had been fired when word came to stand down," the newspaper reported, citing an unnamed senior administration official.
The strikes were set to take place just before dawn on Friday to minimise risk to the Iranian military or to civilians, the newspaper added.
Several other US media outlets then independently reported the same but their accounts differed on the role of the Pentagon.
The Associated Press quoted a US official as saying the strikes had been recommended by the Pentagon and had been among options presented to senior administration officials.
According to the New York Times, top Pentagon officials warned a military response could result in a spiralling escalation with risks for US forces in the region.
The operation was called off after President Trump spent most of Thursday discussing Iran with his national security advisers and congressional leaders, AP reports.
US Secretary of State Mike Pompeo and National Security Adviser John Bolton had pushed for a hardline stance, but congressional leaders urged caution, the agency says.
Democratic House Speaker Nancy Pelosi said the US had no appetite for war with Iran, while the leading candidate for the Democratic presidential nomination, Joe Biden, called Mr Trump's Iran strategy a "self-inflicted disaster".
Senate Majority Leader Mitch McConnell, a Republican, said: "The administration is engaged in what I would call measured responses."
The top Democrat in the US Senate, Chuck Schumer, said: "The president may not intend to go to war here, but we're worried that he and the administration may bumble into a war."
It remains unclear if the apparent strikes could still go ahead.