Iranian officials have urged young people to form human chains to protect power plants, as US President Donald Trump warns a "whole civilization will die tonight" if Tehran does not meet his latest deadline for the Islamic Republic to agree to a deal that includes reopening the crucial Strait of Hormuz.
Meanwhile, airstrikes hit two bridges and a train station in Iran on Tuesday, and the US struck military targets on the Iranian oil hub of Kharg Island. The attack marked the second time the island was hit by American forces.
Trump has extended previous deadlines but suggested the one set for 8pm on Tuesday (10am Wednesday AEST) was final, and the rhetoric on both sides reached a fever pitch, leaving Iranians on edge.
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Trump threatened to destroy all of Iran's power plants and bridges if Tehran does not allow traffic to fully resume in the strait, through which a fifth of the world's oil transits in peacetime.
Iran's president said 14 million people, including himself, have volunteered to fight.
It was not clear if the latest airstrikes were linked to Trump's threat to attack bridges. At least two of the targets were connected to Iran's rail network, which Israel earlier signalled it might attack.
Israel has increasingly carried out strikes that it says are aimed at delivering a blow to Iran's economy.
Iran, meanwhile, fired on Israel and Saudi Arabia, prompting the temporary closure of a major bridge.
While Iran cannot match the sophistication of US and Israeli weaponry or their dominance in the air, its chokehold on the strait is causing major damage to the world economy and raising the pressure on Trump both at home and abroad to find a way out of the standoff.
Officials involved in diplomatic efforts said talks were ongoing – but Iran has rejected the latest American proposal, and it was unclear if a deal would come in time to head off Trump's threatened attacks.
World leaders and experts warned that strikes as destructive as Trump threatened could constitute a war crime.
As the deadline approaches, rhetoric ramps up
"A whole civilisation will die tonight, never to be brought back again," if a deal isn't reached, Trump said in a post on Tuesday morning (night AEST), while keeping open the possibility of an off-ramp, saying that "maybe something revolutionarily wonderful can happen".
Earlier, Iranian official Alireza Rahimi issued a video message calling on "all young people, athletes, artists, students and university students and their professors" to form human chains around power plants.
Iranians have formed human chains in the past around nuclear sites at times of heightened tensions with the West. This time though, it was unclear who would heed the call, and one major power plant in Tehran apparently had been closed off for security purposes at the time the demonstration was to start.
President Masoud Pezeshkian posted on X that 14 million Iranians had answered state media and text message campaigns urging people to volunteer to fight — and said he would join them — while a general from the paramilitary Revolutionary Guard urged parents to send their children to man checkpoints.
The Guard, meanwhile, warned that Iran would "deprive the US and its allies of the region's oil and gas for years" and expand its attacks across the Gulf region if Trump carries out his threat.
In Tehran, the mood was bleak. A young teacher said that many opponents of Iran's Islamic system had hoped Trump's attacks would quickly topple it.
Now, as the war drags on, she fears US and Israeli attacks will spread chaos.
"If we don't have the internet, and if we don't have electricity, water, and gas, we're really going back to the Stone Age, as Trump said," she said told The Associated Press, speaking anonymously for her safety.
Trump's threat prompts warnings of war crimes
French Foreign Minister Jean-Noël Barrot joined a growing chorus of international voices and calling for restraint, saying attacks targeting civilian and energy infrastructure "are barred by the rules of war, international law".
"They would without doubt trigger a new phase of escalation, of reprisals, that would drag the region and the world economy into a vicious circle," the minister said on France Info television.
UN Secretary-General António Guterres also warned the US that attacks on civilian infrastructure are banned under international law, according to his spokesperson.
Such cases are notoriously difficult to prosecute, and Trump told reporters he's "not at all" concerned about committing war crimes.
A wave of airstrikes hits Iran, which fires on Saudi Arabia and Israel
A series of intense airstrikes pounded Tehran, including in residential neighbourhoods. Such strikes in the past have targeted Iranian government and security officials.
Israel's military said it attacked an Iranian petrochemical site in Shiraz, the second day in a row it hit such a facility. Israel also issued a Farsi-language warning telling Iranians to avoid trains throughout the day, likely telegraphing intended strikes on the rail network.
Iranian officials later said that a railway bridge, a train station and a highway bridge had been hit in airstrikes. Neither the United States not Israel immediately claimed the attacks.
Another strike hit the Khorramabad International Airport in western Iran, and an attack on an unidentified target in Alborz province, northwest of Tehran, killed 18 people, according to state media. A total of 15 people were killed in other strikes, Iranian media reported.
Early Tuesday, Tehran launched seven ballistic missiles at Saudi Arabia, which authorities said rained debris near energy facilities as they were intercepted.
The attacks prompted Saudi Arabia to temporarily close the King Fahd Causeway, the only road connection between Bahrain, home to the US Navy's 5th Fleet, and the Arabian Peninsula.
Iran also fired on Israel, with reports of incoming missiles in Tel Aviv and Eilat.
More than 1900 people have been killed in Iran since the war began, but the government has not updated the toll for days.
In Lebanon, where Israel is fighting Iran-backed Hezbollah militants, more than 1400 people have been killed. and more than 1 million people have been displaced. Eleven Israeli soldiers have died there.
In Gulf Arab states and the occupied West Bank, more than two dozen people have died, while 23 have been reported dead in Israel, and 13 US service members have been killed.
Chokehold on the Strait of Hormuz
Iran choked off shipping through the strait after Israel and the US attacked on February 28, starting the war.
That stranglehold and Iran's attacks on the energy infrastructure of its Gulf Arab neighbours have sent oil prices skyrocketing, raising the price of gasoline, food and other basics far beyond the Middle East.
In spot trading Tuesday, Brent crude, the international standard, was above $US108 ($156) per barrel, up around 50 per cent since the start of the war.
On Monday, Tehran rejected a 45-day ceasefire proposal and said it wants a permanent end to the war. But as Trump's deadline neared on Tuesday, an official said indirect communications between the United States and Iran remained underway.
The official said mediators from Pakistan, Egypt and Turkey "are racing against time" to reach a compromise before the deadline.
He said Iran has linked the reopening of the Strait of Hormuz to sanctions relief, and the US was open to easing some sanctions, especially on Iran's oil sector, in part to stabilise the global oil market.
The official spoke on condition of anonymity to discuss ongoing diplomacy.
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