US President Donald Trump said on Tuesday that the United States and Iran had agreed to a two-week ceasefire, announced shortly before a deadline after which he warned that "a whole civilization will die tonight" if Iran did not reopen the Strait of Hormuz.

The 11th-hour deal, brokered by Pakistan's Prime Minister Shehbaz Sharif, was announced by Trump on social media.

Iran's Supreme National Security Council said in a statement: "It was decided at the highest level that Iran will hold negotiations with the American side in Islamabad for two weeks."

Negotiations between the two sides are expected to begin on Friday.

The U-turn by Washington on Iran came as US people had been on edge earlier in the day over Trump's heightened rhetoric.

Trump vowed on Monday that he would destroy Iran's civilian infrastructure by wiping out "every bridge" and power plant if Iran did not strike a deal by his Tuesday deadline. On Tuesday morning, Trump went a step further, threatening that a "whole civilization will die" if Iran did not agree.

Adam Turner, 54, from New York, was tearful about how stressed out the president's rhetoric was making him feel about Iran and a host of other issues since he took office.

"It is without respect. It is without intelligence, without dignity. And the fact that we are represented by that is a crime. So, the way he speaks about Iran is no different from the way he speaks about anything. It's vile.

"It makes me sad because I don't think the Iranian people deserve it," Turner added. "It didn't need to happen. He got rid of (former president Barack) Obama's Iran deal. We had a deal in place that was effective."

Trump's threat shook US citizens, US senators and the United Nations.

A spokesperson for United Nations Secretary-General Antonio Guterres said the UN chief was "deeply troubled" by statements suggesting civilians could bear the consequences of military decisions.

Lewis Fox, 66, from Manhattan, New York, was also disgruntled by the US president's aggressive rhetoric on Iran and wanted to see a more diplomatic approach.

"I think he doesn't have a clue what he's doing. I have no idea how he ended up in a position where he has the ability to do what he is doing.

'Bully of the world'

"He has converted the United States into the bully of the world versus the savior of the world. And therefore, he definitely shouldn't be talking like that."

Other critics of Trump's threatening post toward Iran on Tuesday included former staunch ally Republican Representative Marjorie Taylor Greene. "Not a single bomb had dropped on America," she posted on X. "We cannot kill an entire civilization. This is evil and madness."

House Minority Leader Hakeem Jeffries, a Democrat from New York, on Tuesday called on Republican members of Congress to act.

"Congress must immediately end this reckless war of choice in Iran before Donald Trump plunges us into World War III," Jeffries said in a post on X.

"It's time for every single Republican to put patriotic duty over party and stop the madness. Enough."