Democratic US representative Rashida Tlaib has warned that Donald Trump could "slaughter 100 million people" if a deal is not reached tonight, adding that it "clear he's unfit to be president".

Detroit's U.S. House representatives on Tuesday joined calls from dozens of other Democrats in Congress to remove President Donald Trump from office after he threatened that "a whole civilisation will die tonight" if a deal is not reached with Iran.

Other Democrats from Michigan didn't go as far but also slammed Trump's remarks, calling them outrageous and "morally wrong."

"After bombing a school and massacring young girls, the war criminal in the White House is threatening genocide," Tlaib posted on social media. "This maniac should be removed from office."

Trump had threatened to "slaughter" 100 million people, Thanedar said: "It's clear he's unfit to be president, the 25th amendment must be invoked. If Vance, Rubio & the others continue to be spineless cowards, Congress must do everything possible to stop Trump & this war."

Thanedar also wrote a letter Tuesday to the 16 members of Trump’s cabinet, urging them to act.

Trump vowed on Monday to destroy "every bridge" and power station in Iran in just four hours if a deal isn't reached by 1am.

In response, some Iranians have formed "human chains" on bridges and outside power plants after authorities urged people to gather at potential US and Israeli targets.

The Bulletin of Atomic Scientists has warned that a strike on the Bushehr nuclear power plant would lead to a disaster comparable with Fukushima in 2011, which killed more than 18,500.

Other predictions are more grim.

The International Campaign to Abolish Nuclear Weapons (ICAN) has long warned that even a "limited" nuclear war would trigger a "nuclear winter," blocking sunlight and causing global agricultural collapse and famine for billions.

The White House has denied having active plans for a nuclear strike, though the president's public warnings that a "whole civilisation will die tonight" have kept global tensions at an all-time high.

Modern nuclear warheads are significantly more powerful than the 15-kiloton bomb dropped on Hiroshima, which killed approximately 140,000 people by the end of 1945.

Research published by American physician Frederick M. Burkle predicts that a 500 kt nuclear device dropped on Tehran would killed around 86% of the city's eight million population.

But survivors face acute radiation syndrome and long-term risks like cancer.

Medical experts say that 65% of survivors would be at risk for secondary bacterial infections, complicated by antibiotic resistance.

And horrifying maps show that radioactive plumes from strikes in central Iran could reach Pakistan and northern India within 48 to 72 hours, depending on wind patterns.

Pakistan, which has been acting as a mediator, has called on Trump to extend his deadline for two weeks "to allow diplomacy to run its course".

The Prime Minister of Pakistan Shehbaz Sharif, said: "Diplomatic efforts for peaceful settlement of the ongoing war in the Middle East are progressing steadily, strongly and powerfully with the potential to lead to substantive results in near future.

"To allow diplomacy to run its course, I earnestly request President Trump to extend the deadline for two weeks.

"Pakistan, in all sincerity, requests the Iranian brothers to open Strait of Hormuz for a corresponding period of two weeks as a goodwill gesture.

"We also urge all warring parties to observe a ceasefire everywhere for two weeks to allow diplomacy to achieve conclusive termination of war, in the interest of long-term peace and stability in the region."