Sharp reactions from Tehran, the United Nations and lawmakers in Washington followed US President Donald Trump's threat on Tuesday to wipe out "a whole civilization" if Iran failed to reopen the Strait of Hormuz, heightening tensions at home and abroad.

"A whole civilization will die tonight, never to be brought back again," Trump wrote on X on Tuesday, about 12 hours before his self-imposed 8 pm ET deadline. "I don't want that to happen, but it probably will."

The White House sought to clarify the scope of Trump's remarks, with press secretary Karoline Leavitt saying, "Only the president knows where things stand and what he will do," while noting he was not considering the use of nuclear weapons.

Speaking at a United Nations Security Council meeting, Iran's UN envoy Amir Saeid Iravani said such rhetoric was "unfitting of any political leader, let alone the head of a permanent member of the Council entrusted with the maintenance of international peace and security."

Iranian President Masoud Pezeshkian responded with defiance, saying public support for defending the country remained strong and signaling Tehran would not yield under pressure.

"More than 14 million proud Iranians have so far registered to sacrifice their lives to defend Iran. I too have been, am, and will remain devoted to giving my life for Iran," he wrote on X.

UN Secretary-General António Guterres on Tuesday called for increased diplomatic efforts to end the conflict.

"The Secretary-General reiterates that conflicts end when leaders choose dialogue over destruction. That choice still exists, and it must be made now," Stéphane Dujarric, the UN spokesperson, told reporters in response to questions about recent remarks made against Iran.

He added that Guterres is urging stepped-up diplomacy to find a peaceful path forward and has dispatched his personal envoy, Jean Arnault, to the region to support those efforts.

The UN issued a reminder of international humanitarian law amid the escalating rhetoric.

"Even wars have rules," the UN wrote in a post on X on Tuesday. "The Geneva Conventions protect civilians in conflict and help ensure assistance reaches those in need, without discrimination."

In Washington, criticism emerged from across the political spectrum, with several Democratic lawmakers warning that targeting civilian infrastructure would constitute a war crime.

Democratic Senator Chris Murphy from Connecticut said: "GOP leaders need to stop him. Never mind that blowing up bridges and power plants and killing innocent Iranians won't reopen the Strait. It's also a clear war crime."

Democratic Senator Ed Markey of Massachusetts described the situation as "completely unstable and perilous."

"The House must bring up impeachment articles, and the Senate needs to remove a president who wants to commit war crimes. We cannot sit idly by as Donald Trump threatens to end an entire civilization," he said.

House Democratic leader Hakeem Jeffries noted the broader consequences.

"Congress must immediately end this reckless war of choice in Iran before Donald Trump plunges us into World War III," Jeffries said. "It's time for every single Republican to put patriotic duty over party and stop the madness."

Senate Minority Leader Chuck Schumer also urged Republican lawmakers to intervene, calling Trump an "extremely sick person."

"Each Republican who refuses to join us in voting against this wanton war of choice owns every consequence of whatever the hell this is," Schumer said.

Criticism also came from within Republican circles. Former Republican Representative Marjorie Taylor Greene said: "Not a single bomb had dropped on America. We cannot kill an entire civilization. This is evil and madness."

The US and Israel launched new strikes across Iran on Tuesday, including energy hub Kharg Island.

Canadian Prime Minister Mark Carney also said that international law should be followed and civilian targets must not be hit.

"Canada expects all parties in this conflict, in any conflict, to respect international laws," Carney said during a news conference on Tuesday.

"That means not targeting civilians or civilian infrastructure. And we urge all parties in this war to follow those responsibilities," he said.

Pakistan, acting as a mediator between the warring countries, has proposed that Trump extend his deadline by two weeks and that Iran should reopen the Strait of Hormuz for that period.

The White House says it's reviewing the proposal.

Agencies contributed to this story.

gaoyang@chinadailyusa.com