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.
EXCLUSIVE: In an exclusive interview with Chuck Todd, President Donald Trump says he hadn’t given final approval to Iran strikes, no planes were in the air. pic.twitter.com/qHtTx7Zql5— Meet the Press (@MeetThePress) June 21, 2019
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.
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.
President Trump's decision to order – and then abort – an attack still sends a powerful message to Tehran.
The two countries came to the brink of direct conflict. But in this complex game of signalling, just what message will the Iranian leadership receive?
It, after all, had sent a significant warning of its own by downing an unmanned US reconnaissance drone.
Mr Trump initially appeared to play down the incident – but then apparently came the orders for a US retaliatory strike. That was followed by a last-minute change of heart.
The danger now is that Iran receives mixed messages that convey uncertainty and lack of resolve. This might encourage some in Tehran to push back at the Americans even harder.
There appears to be no diplomatic "off-ramp" in this crisis.
US economic sanctions are hitting home. Tehran is under pressure. Escalation remains an ever-present danger.
What happened with the drone?
Iran's Islamic Revolutionary Guard Corps (IRGC) announced its air force had shot down a US "spy" drone on Thursday morning after the unmanned aircraft violated Iranian airspace near Kuhmobarak in the southern province of Hormozgan.
IRGC commander-in-chief Maj-Gen Hossein Salami said the drone's downing was a "clear message" to the US that Iran's borders were "our red line".
However, US military officials maintain the drone was in international airspace over the Strait of Hormuz at the time.
Iranian officials say two warnings were issued 10 minutes before the drone was shot down.
Amir Ali Hajizadeh, a high-ranking officer in the IRGC, said another military aircraft, carrying 35 passengers, had been flying close to the drone. "We could have shot down that one too, but we did not," he said.
The shooting down of the drone followed accusations by the US that Iran had attacked two oil tankers with mines last Thursday just outside the Strait of Hormuz, in the Gulf of Oman.