The United States and Israel stepped up their attacks ahead of the 8 p.m. Eastern deadline Mr. Trump had set for Iran to allow unimpeded shipping through the strait, a key oil and gas shipping route. He issued his grave warning on social media as U.S. forces struck Kharg Island, the oil export hub, as part of a broader effort to force Iran to make a deal, according to American and Israel officials with knowledge of the operations, who spoke on the condition of anonymity to discuss sensitive matters.

“A whole civilization will die tonight, never to be brought back again,” the American president wrote, adding that he hoped “maybe something revolutionarily wonderful can happen” to avoid the attacks. “We will find out tonight, one of the most important moments in the long and complex history of the World.”

On Monday, the president said at a news conference that if Iran did not end its effective blockade of the strait, every bridge in the country would be “decimated,” and every power plant would be “out of business.”

The Israeli military said it had launched airstrikes on eight bridges across Iran, and warned Iranians not to ride railroads until 9 p.m. local time. Iranian state media reported that at least three people were killed when a railway bridge was hit in the central city of Kashan.

Striking civilian infrastructure could be a war crime under international law. Ebrahim Zolfaghari, a spokesman for the Iranian military, has said that Iran would retaliate “crushingly and extensively” if its civilian infrastructure were attacked.

Some Iranians were bracing for the possibility of more strikes, while others were responding to the American threats with a mix of indifference, defiance, and bewilderment. “I think Trump is under a lot of pressure and that he has lost his mind,” said Lili, a Tehran resident who asked not to use her full name out of concern for repercussions for speaking to foreign media. She and her family were not planning to flee the city, she added, because there was nowhere to go.

Iran targeted energy facilities in Persian Gulf countries allied with the United States. Saudi Arabia’s defense ministry said that debris from an intercepted missile had fallen near energy facilities in the east and that the damage was being assessed. The United Arab Emirates’ defense ministry said missiles and drones had been fired from Iran.

Negotiations between Iran and the United States had been mediated by Pakistan and other regional allies, which had proposed a 45-day cease-fire. On Monday, before withdrawing from talks, Iran had delivered a separate 10-point plan to end the war to the United States and Israel through Pakistan, according to Iranian state media.

Here’s what else we’re covering:

• Oil prices: Oil prices initially climbed on Tuesday after Mr. Trump rejected a cease-fire proposal the previous day as “not good enough.” The price of Brent Crude, the international benchmark, rose 1.5 percent to about $111 a barrel, before falling slightly.

• Strait of Hormuz: Britain will host a virtual meeting on Tuesday of military planners from more than 40 countries about how to secure safe passage through the waterway after the war ends.

• Pentagon media: The Pentagon canceled a news conference with Defense Secretary Pete Hegseth and Dan Caine, the chairman of the Joint Chiefs of Staff, that was scheduled for Tuesday.

• Death tolls: The Human Rights Activists News Agency said at least 1,606 civilians, including 244 children, had been killed in Iran as of Friday. Lebanon’s health ministry on Thursday said at least 1,345 Lebanese had been killed in the latest fighting between Israel and Hezbollah. In attacks blamed on Iran, at least 50 people have been killed in Gulf nations. In Israel, at least 20 people had been killed as of Monday. The American death toll stands at 13 service members, with hundreds of others wounded