US President Donald Trump has threatened that "a whole civilization will die tonight" as Iran shows no sign of accepting his ultimatum to open the Strait of Hormuz.
He has given Iran a deadline of 1am Irish time tomorrow to end its blockade of Gulf oil or every bridge and power plant in the country will be destroyed.
Iran said it would retaliate against US allies in the Gulf, whose desert cities would be uninhabitable without power or water.
As the clock ticked down, 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 Mr Trump has openly spoken about seizing.
Iran responded by declaring that it would no longer hold back from hitting its Gulf neighbours' infrastructure.
It also claimed to have carried out fresh strikes on a ship in the Gulf and a huge Saudi petrochemical complex.
"A whole civilization will die tonight, never to be brought back again. I don't want that to happen, but it probably will," Mr 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.
"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."
It was not clear exactly what he meant by his latest threat, or by what means he intended to carry it out.
Hours ahead of the deadline, Iranian First Vice President Mohammad Reza Aref said his government was prepared "for all scenarios".
"No threat is beyond our preparedness and intelligence," he added.
Both Mr Trump and Iran have turned down a proposal touted by international mediators for a 45-day ceasefire.
Infrastructure attacks reported by Iranian authorities Tuesday included a US-Israeli strike on a bridge outside the city of Qom and another on a rail bridge in central Iran that killed two people.
Regional authorities also said a US-Israeli strike shut down a key highway in northern Iran connecting the city of Tabriz with Tehran.
The Mizan news agency additionally reported a strike on railway tracks in Karaj, outside Tehran.
Death 'not a joke'
University student Metanat, whose classmate was killed two weeks ago in an attack, said she felt "terrified and so should everyone else in the country".
The 27-year-old, who declined to give her last name, said as far as Mr Trump's ultimatums were concerned, "some people think they are a joke", but "death is not a joke".
The UN's rights chief decried the "incendiary rhetoric" in the Middle East war, warning that deliberate attacks on civilians and civilian infrastructure was "a war crime".
Pope Leo called the threat against Iran's population "truly unacceptable", saying that in addition to raising questions under international law, it was "a moral question".
At the UN Security Council, Russia and China vetoed a resolution on reopening the Strait of Hormuz, a text already diluted to remove the green light Gulf states had sought to use force to protect the key shipping lane.
Iran has effectively blocked the waterway - through which a fifth of the world's oil normally flows - since the start of the war on 28 February, driving up global energy prices.
Iranian pensioner Morteza Hamidi said that among many emotions, he felt "gloomy for the future of the country after the war".
The 62-year-old added that he had seen Mr Trump back down too many times to take his words seriously:
"We are now numb to his threats".
State media, meanwhile, published photos purporting to show groups of Iranians forming human chains to protect power plants as the hours to the deadline ticked down.
Kuwait, meanwhile, urged its citizens to remain indoors from midnight until 7am, while Bahrain's main port said it would be suspending operations starting early Wednesday.
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Other strikes
Earlier in the day a series of explosions was heard across Tehran, and Iranian media reported that 18 people, including two children, were killed in strikes in neighbouring Alborz province.
US-Israeli strikes also "completely destroyed" the capital's Rafi-Nia synagogue, local media reported. The Israeli army later expressed regret for the "collateral damage", saying it had been targeting a senior military commander, not the house of worship.
Overnight, attacks on Saudi Arabia hit a petrochemical complex in a sprawling industrial area in the eastern city of Jubail, a witness who requested anonymity said, hours after similar installations in Iran were struck.
Gulf states have faced the brunt of Iran's retaliatory attacks without responding, though Bahraini Foreign Minister Abdullatif bin Rashid Al Zayani warned that "this restraint cannot be expected to continue without limit".
On the war's Lebanese front, the Israeli military said it had completed deployment of ground troops along a "defence line" in the south, where it is fighting Iran-backed Hezbollah.
It additionally urged all vessels in the maritime zone off the country's south to immediately head north of the city of Tyre, warning that it would operate in the area.
