US President Donald Trump on Friday said the US military was "cocked and loaded to retaliate" against Iran, but he changed his mind 10 minutes before planned strikes.
He had initially approved attacks on several Iranian targets such as radar and missile batteries, Xinhua news agency quoted a New York Times report as saying citing administration officials.
Trump, however, on Thursday night called off the operation that had been underway in its early stages, a senior administration official said.
Attacks on three sites were planned in response to the shooting down of a US unmanned drone this week.
Trump said he had called off the strikes after being told that around 150 people could die. He tweeted: "10 minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone".
"I am in no hurry," Trump said on Friday. "Our military is rebuilt, new, and ready to go, by far the best in the world."
He tweeted that Iran had downed the drone on Monday, despite an earlier US military statement that the incident happened on Wednesday night.
Tehran said the unmanned US aircraft entered Iranian airspace early on Thursday morning. Washington, however, maintains it was shot down in international airspace.
Tension has 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 internationally agreed limits on its nuclear programme.
In 2018, the US unilaterally pulled out of a 2015 nuclear deal aimed at curbing Iran's nuclear activities and also resumed "maximum" sanctions against Tehran.
"Iran can never have nuclear weapons," Trump said in his tweets on the aborted strikes - also revealing that increased economic sanctions against Iran were "added last night".
The US military had confirmed on Thursday that a RQ-4A Global Hawk Unmanned Aircraft System (UAS) was shot down by an Iranian surface-to-air missile system on Wednesday night.
Iran, however, claimed the drone downed in the southern part of the country had crossed Iranian borders.
"Iran made a very bad mistake," Trump told reporters, adding that the shooting down of the drone was "a new wrinkle" that the US would not tolerate.
Earlier incidents including several attacks on oil tankers in the region in the past few weeks also took a toll on the worsening ties.