"A whole civilisation will die tonight, never to be brought back again. I don't want that to happen, but it probably will," Trump declared.

Iran has a population of around 90 million people, all of whom Trump has threatened to kill.

Trump, however, said that "smarter and less radicalised" elements within the regime could accept his demands and avoid that fate.

Trump has given Iran until 8 pm ET to accept his terms or face annihilation. This is the second time in 24 hours that he has threatened to kill all Iranians if the government does not accept his demands.

Previously, Trump on Monday declared that the "entire country can be taken out in one night, and that night might be tomorrow night".

"Every power plant will be out of business, burning, exploding and never to be used again. I mean complete demolition by 12 o'clock. It will happen over a period of four hours if we want it to," Trump further said.

US strikes Kharg Island as deadline nears

Trump's latest threat came shortly after it emerged that the United States had struck Iran's Kharg Island — the country's main petroleum export hub.

A US official told Fox News that the military struck bunkers, radar station, and ammunition storage in overnight strikes on the island.

Kharg Island is widely believed to be one of the potential targets of any US-Israeli ground offensive. Trump has long flirted with the idea of capturing the island's oil and gas stockpile — around 90 per cent of Iran's oil and gas is exported from the island.

Previously, the United States had bombed military facilities on the island on March 13-14.