US military was 'cocked and loaded' to hit Iran: Trump

President Donald Trump on Friday said the United States military was "cocked and loaded" to retaliate against Iran after it downed an American spy drone, but he changed his mind 10 minutes before the planned strikes on three Iranian targets after being told that 150 people could die.

The late reversal was first reported by the New York Times on Thursday night after President Trump tweeted: "Iran made a very big mistake!"

The newspaper said the operation had been "in its early stages" when President Trump called off the US military action against Tehran.

On Friday, Trump confirmed the report and tweeted: "10 minutes before the strike I stopped it, not proportionate to shooting down an unmanned drone".

"I am in no hurry," Trump said. He said he had called off strikes after being told that 150 people would die.

The attacks on three Iranian sites were planned in response to the shooting down of a US unmanned drone this week. Iran said Thursday that its elite Revolutionary Guard shot down an RQ-4 Global Hawk drone as it flew over southern Iran and that the incident sent a "clear message to America".

Tehran says the unmanned US aircraft entered Iranian airspace. The US maintains it was shot down in international airspace.

The aborted US strikes against Iran on Thursday were meant to target a limited set of Iranian radars and missile batteries, CNN quoted a US official with direct knowledge of the matter as saying.

The strikes were set to begin on Thursday night when the White House called them off, said the official, who spoke on the condition of anonymity because of the sensitivity of the information.

No US weapons had been launched when the decision was made to call the strikes off, the official said.

Tensions have been escalating between Washington and Tehran, with the US recently blaming Iran for attacks on oil tankers operating in the region.

Iran has also 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.

"Iran can NEVER have Nuclear Weapons, not against the USA, and not against the WORLD!" Trump said in his tweets on the aborted strikes - also revealing that increased economic sanctions against Iran were "added last night".

In its initial report, The New York Times said that as late as 19:00 local time on Thursday, US military and diplomatic officials had still expected the strikes to take place on agreed targets, including Iranian radar and missile batteries.

"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 had been set to take place just before dawn on Friday to minimise risk to the Iranian military or to civilians, the newspaper added.

In the US, Democratic House Speaker Nancy Pelosi said the US had no appetite for war with Iran, while the leading candidate for the Democratic presidential nomination in 2020, Joe Biden, called Trump's Iran strategy a "self-inflicted disaster".