• The US has now asked the UN Security Council to meet on Monday to discuss Iran.
• Tensions have been escalating between the two countries, with the US recently blaming Iran for attacks on oil tankers operating in the region.
President Trump has said he does not want war but warned Iran it would face "obliteration" if conflict broke out.
Speaking to NBC on Friday, he said the US was open to talks but would not allow Iran to develop nuclear weapons.
He also expanded on his last-minute decision to call off strikes planned in response to the shooting down of a US unmanned drone this week, saying he had been told 150 Iranians would be killed.
"I didn't like it. I didn't think it was proportionate," he said.
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.
The US has now asked the UN Security Council to meet on Monday to discuss Iran.
What did Trump tell NBC?
He said a plan of attack was "ready to go, subject to my approval" but said he had then asked generals how many people would be killed.
"I thought about it for a second and I said, you know what, they shot down an unmanned drone, plane, whatever you want to call it, and here we are sitting with 150 dead people that would have taken place probably within a half an hour after I said go ahead," he told NBC.
He denied suggestions that aircraft had already been on their way to attack Iranian targets - reportedly including Iranian radar and missile batteries - saying: "No planes were in the air."
Addressing Iran's leaders, Mr Trump said: "You can't have nuclear weapons. And if you want to talk about it, good. Otherwise, you can live in a shattered economy for a long time to come."
Earlier on Friday Mr Trump tweeted that the US had been "cocked and loaded" to strike.
What reaction has there been?
Democratic House Speaker Nancy Pelosi said she was glad the president had not carried out the strikes and said he should seek congressional authorisation before military action.
Adam Smith - the Democratic chair of the House Armed Services Committee - warned it was "not smart" of the president to make the details public, saying it undermined the notion of a clear US plan.
Some US media reports said the strikes had been recommended by the Pentagon, while others said top Pentagon officials had warned a military response could result in a spiralling escalation with risks for US forces in the region.
US Secretary of State Mike Pompeo and National Security Advisor John Bolton had pushed for a hardline stance, but congressional leaders had urged caution, the Associated Press reported.
The US Federal Aviation Administration (FAA) issued an emergency order on Thursday evening prohibiting US airlines from operating in an overwater area of Tehran-controlled airspace nearby in response.
Airlines from other countries, including Britain's British Airways, the Dutch carrier KLM, Emirates, and Qantas of Australia, have also said they will re-route their flights to avoid parts of Iran.
Meanwhile, a UK government minister will hold talks with Iranian officials in Tehran on Sunday.
Andrew Murrison will call for "urgent de-escalation" in the region, the Foreign Office said.
What does Iran say?
An official warned that "any attack against Iran will have regional and international consequences".
"When you violate Iranian territorial space, then we defend," Seyed Sajjadpour, one of Iran's deputy foreign ministers, told the BBC.
It was clear, he added, that there were members of Donald Trump's administration who were intent on overthrowing Iran's government.