Washington DC - Donald Trump is the only person who knows his plans for Iran, the White House said, after the president warned that a "whole civilization will die" if Tehran fails to reach a deal Tuesday.

"The Iranian regime has until 8 PM Eastern Time to meet the moment and make a deal with the United States," Press Secretary Karoline Leavitt said in a statement to AFP, when asked if Trump was prepared to use a nuclear weapon, and about a report that Iran had cut off negotiations.

"Only the President knows where things stand and what he will do."

Earlier in the day, the White House denied that Vice President JD Vance implied the US was considering nuclear strikes after he claimed American forces had tools they "so far haven't decided to use" to enforce Trump's ultimatum.

Donald Trump Trump extends his Iran deadline after hurling threats in expletive-laden post

"Literally nothing @VP said here 'implies' this, you absolute buffoons," the White House said on X.

The post was in response to one from an account associated with former vice president Kamala Harris that said Vance implied Trump "might use nuclear weapons."

Trump's shocking threats against Iran's critical infrastructure, and now the country at large, also sparked fierce condemnation from the UN on Tuesday.

"Under international law, deliberately attacking civilians and civilian infrastructure is a war crime," UN high commissioner for human rights Volker Turk said in a statement, insisting that "anyone responsible for international crimes must be held to account by a competent court."