US President Donald Trump threatened that "a whole civilization will die tonight" as Iran showed no sign of accepting his ultimatum to open the Strait of Hormuz by Tuesday evening, Washington time.

Trump has given Iran until 8 pm in Washington - 3:30 am in Tehran (5.30 am IST) - to end its blockade of Gulf oil or see the US destroy every bridge and power plant in Iran. Iran says it would retaliate against US allies in the Gulf, whose desert cities would be uninhabitable without power or water.

As the clock ticked down on Trump's deadline, strikes on Iran intensified throughout the day, 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, which Trump has openly mused about seizing.

Iran responded by declaring it would no longer hold back from hitting its Gulf neighbours' infrastructure, and claimed to have carried out fresh strikes on a ship in the Gulf and a huge Saudi petrochemical complex.

Trump's threats reach new level
"A whole civilization will die tonight, never to be brought back again. I don’t want that to happen, but it probably will," Trump wrote on his Truth Social website, in a statement directed at a nation that takes pride in being one of the earliest centres of civilisation, dating back thousands of years into antiquity.

"However, now that we have Complete and Total Regime Change, where different, smarter, and less radicalized minds prevail, maybe something revolutionarily wonderful can happen, WHO KNOWS? We will find out tonight, one of the most important moments in the long and complex history of the World."

Brian Finucane, a former US State Department legal advisor 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.

With only hours left before the deadline, a senior Iranian source said Tehran was maintaining its refusal to reopen the strait without US concessions that so far were not forthcoming.

Pakistan, which has been the main go-between, was still relaying messages, but Washington had not changed its tone, the source said. If the US carried out Trump's threat to hit Iran's power grid, Tehran would plunge Gulf states including Saudi Arabia into darkness, the source added, a threat that had been conveyed to Washington via Qatar.

Earlier, another senior Iranian source told Reuters that Tehran had rejected a proposal conveyed by intermediaries for a temporary ceasefire.

Talks on a lasting peace could begin only after the US and Israel stop bombing, guarantee not to start again and offer compensation for damage, the Iranian source said, adding that any settlement must leave Iran in control of the strait, imposing fees for transit.

Despite the intensification of strikes and rhetoric from both sides, global markets were largely paralysed, hesitant to bet on whether Trump would follow through on his threats or call them off as he has in the past.

Israel launched fresh attacks on Iranian infrastructure ahead of Trump's deadline. It targeted train tracks and bridges that Prime Minister Benjamin Netanyahu said had been used by the Revolutionary Guards to transport operatives, weapons and raw materials. He provided no evidence to support his claims.

It also warned Iranians in a Persian-language social media post that anyone near railways would be in danger.

Power was knocked out in parts of Karaj west of Tehran by a strike on transmission lines and a substation.

A synagogue in Tehran was destroyed overnight by what Iran said were Israeli air strikes. Footage in Iranian media showed Hebrew texts scattered in the debris.

"The synagogue building was completely destroyed and our Torah scrolls were left under the rubble," said Homayoun Sameh, a lawmaker representing Iran's Jewish community, one of the Middle East's largest outside Israel. Israel's military had no immediate comment.

Pakistan tries to broker peace
Iran responded to an overnight attack on a major petrochemical site with a strike on Saudi Arabia's huge downstream oil industry site at Jubail, where Western oil firms operate multi-billion dollar ventures. Video verified by Reuters showed smoke and flames rising.

Iran's Revolutionary Guards said in a statement that Tehran would "deprive America and its allies in the region of oil and gas for years".

"Up to today we have shown great restraint for the sake of good neighbourliness and have had some consideration in choosing targets for retaliation," it said. "But all these restraints have since been removed."

Iranians hoped the threatened escalation could be averted.

"I hope it is another bluff by Trump," Shima, 37, from the central city of Isfahan, told Reuters by phone.

Trump has abruptly called off similar threats over the past several weeks, citing what he has described as productive negotiations with figures in Iran he has never identified. Tehran has denied any such substantive talks have taken place.

Iran's ambassador to Pakistan said "positive and productive endeavours" by Islamabad to mediate an end to the war were "approaching a critical, sensitive stage".

A proposal conveyed by Pakistan called for a temporary ceasefire and the lifting of Iran's effective blockade of the strait, while putting off a broader peace settlement for further talks, according to a source familiar with the plan.

But Iran's 10-point response, as reported by IRNA news agency on Monday, would require a permanent end to the war, the lifting of sanctions and a promise of reconstruction of Iranian sites damaged by the Israeli-US strikes.

It would also include a new mechanism to govern passage through the strait - previously an open international waterway through which a fifth of global oil and liquefied natural gas typically passed. Since the US and Israel attacked Iran on February 28, Iran has effectively closed it to most ships.