Trump however offered no firm plan for Iran, saying possible leaders eyed by the US had been killed and admitting that a replacement for slain supreme leader Ali Khamenei could be just as bad.

The 79-year-old Republican has faced criticism for conflicting messages about his justifications and aims after launching the region's biggest conflict in more than two decades.

"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.

"They have no navy, it's been knocked out. They have no air force. It's been knocked out. They have no air detection, that's been knocked out. Their radar has been knocked out."

Trump, who has given differing reasons in recent days for launching "Operation Epic Fury," claimed that Iran was going to strike first and that the United States acted to pre-empt it.

In doing so, he walked back Secretary of State Marco Rubio's comments on Monday that Washington did so only after learning that ally Israel was going to strike.

"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."

Trump faced criticism for the lack of an apparent plan for Iran, and he admitted that he was not sure how the situation would play out.

"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.

- New Iran leadership candidates 'dead'-

He said that two waves of US-Israeli attacks had killed Iranian 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."

Trump has previously urged the people of Iran to rise up and overthrow their government, but toppling the Islamic republic was not among the four key goals for the operation that he gave on Monday, including stopping its nuclear program.

On Tuesday the US leader told protesters to hold off. He has also used the recent deadly crackdown on protests in Iran as a justification for the war.

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

Germany's Merz Chancellor Friedrich Merz voiced support Tuesday for the US-Israeli war on Iran but said he hoped it would end soon, saying it was hurting the global economy.

"This is, of course, damaging our economies. This is true for the oil prices, and this is true for the gas prices as well. So that's the reason why we all hope that this war will come to an end as soon as possible," he said.

Trump insisted soaring oil prices would drop "lower than even before" after the war ends.

While praising Merz, Trump had harsh words for European allies Britain and Spain.

"This is not Winston Churchill that we're dealing with," Trump said of British Prime Minister Keir Starmer who initially refused to let US forces use UK bases, before relenting.

Trump also threatened to cut off trade with Spain, whose left-wing government refused to let US planes use its bases to attack Iran and objected to raising defense funding as part of NATO.

"Spain has been terrible," Trump said, adding: "We could use their base if we want. We could just fly in and use it."