"No, I wouldn't use it," Trump told reporters at the White House.

"Why would I use a nuclear weapon when we've, in a very conventional way, decimated them without it?" he asked.

"A nuclear weapon should never be allowed to be used by anybody."

Trump on April 7 issued a genocidal threat to Iran that a "whole civilization will die tonight, never to be brought back," but within hours agreed to a ceasefire that he has since extended in the war launched by the United States and Israel.

Vice President JD Vance during the conflict warned that the United States was ready to intensify damage on Iran with weapons not previously used, but the White House denied he was threatening nuclear strikes.

Vance in failed negotiations had pushed Iran for greater concessions on its contested nuclear work.

Trump told reporters that he was seeking an Iran "without a nuclear weapon that's going to try and blow up one of our cities or blow up the entire Middle East."

Iran denies seeking a nuclear weapon and the UN nuclear watchdog says that an atomic bomb was not imminent before the war.

The United States is the only country to have used nuclear weapons in combat, obliterating the Japanese cities of Hiroshima and Nagasaki at the end of World War II, killing some 214,000 people.

Israel is widely known to have nuclear weapons but does not publicly acknowledge them.

Trump's blanket statement against any nuclear use would appear to be at odds with longstanding US nuclear doctrine, which reserves the right to use nuclear weapons.

Trump has previously called for an end to a US moratorium on nuclear testing in response to US allegations of secret testing by China and Russia.

Former president Barack Obama had called for an eventual goal of a world without nuclear weapons, but his administration also said that so long as they existed, the US arsenal would serve as a deterrent.

The United States has rejected calls to declare that it will never use nuclear weapons first in a conflict.