Hours before the US president's deadline of 8pm EDT Tuesday (10am AEST Wednesday), Tehran was positively reviewing Pakistan's request to secure more time for diplomacy, a senior Iranian official told Reuters. The White House said Trump was aware of the proposal and would respond.
Trump's extraordinary threat to destroy every bridge and power plant in Iran unless it ends the blockade of Gulf oil unnerved global leaders, rattled financial and energy markets and drew widespread condemnation, including from the head of the United Nations and Pope Leo.
Some international law experts have said a widespread attack on civilian infrastructure could constitute a war crime.
As the clock ticked down, US and Israeli strikes on Iran intensified, hitting railway and road bridges, an airport and a petrochemical plant. US forces attacked targets on Kharg Island, home to Iran's main oil export terminal.
Iran responded by declaring it would no longer hold back from hitting its Gulf neighbours' infrastructure and said it had carried out fresh strikes on a ship in the Gulf and a huge Saudi petrochemical complex.
Booms were heard in Doha late on Tuesday night, according to a Reuters witness in the Qatari capital.
The war, now in its sixth week, has left more than 5000 people dead in nearly a dozen countries, including more than 1600 civilians in Iran, according to tallies from government sources and human rights groups.
Prime Minister Shehbaz Sharif of Pakistan, which is mediating between the two sides, said in a post on social media platform X that Trump should extend his deadline by two weeks and Iran should open the Strait of Hormuz for two weeks as a goodwill gesture.
The state of negotiations was unclear, with Trump and Iranian leaders exchanging bellicose threats.
An Iranian official told Reuters about five hours before the deadline that the US and Iran were still exchanging messages via mediators, without providing details.
"A whole civilisation will die tonight, never to be brought back again. I don't want that to happen, but it probably will," Trump wrote on Truth Social.
"However, now that we have Complete and Total Regime Change, where different, smarter, and less radicalised minds prevail, maybe something revolutionarily wonderful can happen, WHO KNOWS?"
United Nations Secretary-General Antonio Guterres was "deeply troubled" by Trump's statement, his spokesperson said.
Pope Leo said threats against the population of Iran are "unacceptable". Iran's UN ambassador, Amir Saeid Iravani, called Trump's threat "deeply irresponsible" and "profoundly alarming".
Speaking at a UN Security Council meeting where China and Russia vetoed a resolution aimed at protecting commercial shipping through the Strait of Hormuz, Iravani said Trump's "rhetoric is unfitting of any political leader".
The Israeli military said there was a possibility of increased fire towards Israel as Trump's ultimatum nears. It said it was ready to operate both "defensively and offensively".
Iranians were also watching the clock in hope of a reprieve. "I hope it is another bluff by Trump," Shima, 37, from the central city of Isfahan, told Reuters by phone.
Brian Finucane, a former US State Department legal adviser now with the International Crisis Group, said Trump's remarks "could plausibly be interpreted as a threat to commit genocide" under US and international law.
