A day after sounding conciliatory and suggesting a deal could be reached this week, Trump wrote on his Truth Social network that the United States is in "serious discussions" with "a more reasonable regime" in Tehran.

But he added an ominous warning.

"If for any reason a deal is not shortly reached, which it probably will be, and if the Hormuz Strait is not immediately 'Open for Business,' we will conclude our lovely 'stay' in Iran by blowing up and completely obliterating all of their Electric Generating Plants, Oil Wells and Kharg Island (and possibly all desalinization plants!)," Trump said.

White House Press Secretary Karoline Leavitt repeated the administration's insistence that the US war against Iran should be over within another two weeks.

Trump "has always stated four to six weeks, estimated timeline," Leavitt told reporters. "We're on day 30 today. So again, you do the math."

"He wants to see a deal over the next 10 days," she said.

Asked whether Trump's threat to devastate Iranian civilian infrastructure would not risk committing war crimes, Leavitt said the US armed forces would always act within the law.

However, she warned Iran that the US military "has capabilities beyond their wildest imagination and the president is not afraid to use them."

On Sunday night, Trump told reporters on Air Force One that the United States had achieved "regime change" in Iran through the war launched a month ago with Israel, citing the number of Iranian leaders who have been killed. He called the new leadership "much more reasonable".

"We're dealing with different people than anybody's dealt with before. It's a whole different group of people. So I would consider that regime change."

There could be a deal "soon," Trump said when asked if an agreement could come this week.

Meanwhile, when asked if US allies in the Gulf -- like Kuwait, Saudi Arabia and United Arab Emirates -- should help pay for the costs incurred by the war, Leavitt said Monday it was something Trump was "quite interested in doing."

"It's an idea that I know he has," she said without further detail.