"Just about everything's been knocked out," Trump said as he met German Chancellor Friedrich Merz, answering his first questions from reporters since the strikes began on Saturday.

Trump's administration has given conflicting reasons for going to war, with Secretary of State Marco Rubio saying Monday that the United States did so only after learning that ally Israel was going to strike.

But Trump walked that back, saying that he acted to prevent Tehran launching an assault first.

"Based on the way the negotiation was going, I think they (Iran) were going to attack first. And I didn't want that to happen," Trump said in the Oval Office.

"So, if anything, I might have forced Israel's hand."

The US president has also faced criticism for the lack of an apparent day-after plan for Iran, and he admitted that he was not sure how the situation would play out after the conflict.

"I guess the worst case would be, we do this, and then somebody takes over who's as bad as the previous person, right?" Trump said, referring to Ayatollah Ali Khamenei.

Iran's supreme leader was killed in air strikes on the first day of the conflict.

"I guess the worst case would be, we do this, and then somebody takes over who's as bad as the previous person, right?" said Trump.

"That could happen. We don't want that to happen."

Trump also told protesters in Iran to hold off any major moves until the situation had stabilized. He has also used the recent deadly crackdown on protests in Iran as a justification for the war.

"We've said, don't do it yet. If you're going to go out and protest, don't do it yet," Trump added.

He said that two waves of US-Israeli attacks on Iran had killed figures he had eyed as potential new leaders, adding that there had been a "substantial" new attack on a meeting to choose the new leadership.

"Most of the people we had in mind are dead," he said. "Now we have another group. They may be dead also, based on reports."