"A whole civilization will die tonight, never to be brought back again. I don't want that to happen, but it probably will," Trump wrote on his Truth Social platform.

Trump did not give details but he has already said the US military could bomb Iran's bridges, power plants and other civilian infrastructure into the "stone age."

Trump has announced a deadline of midnight GMT Tuesday for Iran to end its de facto closing of the Strait of Hormuz, a narrow, strategic waterway used to transport oil and other commodities.

On Monday, he said a temporary cease-fire proposal being circulated was insufficient.

Iran has rejected US pressure, with state media reporting that the authorities are insisting that instead of just a cease-fire it wants a full end to the war.

In his Truth Social post, Trump left the door open for a last-hour agreement.

"Now that we have Complete and Total Regime Change, where different, smarter, and less radicalized minds prevail, maybe something revolutionarily wonderful can happen, WHO KNOWS? We will find out tonight," he wrote.

Trump said Monday that starting late evening in Washington, DC on Tuesday, US forces would destroy "every bridge in Iran" and cripple "every power plant" in the country -- something that many experts say would be a blatant war crime.

US and Israeli warplanes have been continuously bombing Iran since February 28 and on Tuesday, even before Trump's threat about ending "civilization," Tehran said that its crucial oil export terminal on Kharg island was under attack.