Washington DC - President Donald Trump on Monday said he is not ruling out sending US troops into Iran, while threatening a new, "big wave" of attacks.

The 79-year-old Republican has long campaigned against decades of US military entanglements in the Middle East, but ordered a large-scale war against Iran starting Saturday.

While so far the assault on Iran has focused entirely on aerial attacks by missiles and bombs, Trump refused to rule out sending ground troops – something far riskier in terms of possible casualties.

"I don't have the yips with respect to boots on the ground," Trump said, using a golf term for anxiety. "Every president says, 'There will be no boots on the ground.' I don't say it."

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"I say 'probably don't need them,' [or] 'if they were necessary,'" he told the New York Post in one of numerous brief interviews he has given since launching the Iran operation.

Trump also spoke to CNN on Monday, flagging what he said would be an escalation in the assault on Iran.

"We haven't even started hitting them hard. The big wave hasn't even happened," he told CNN, without elaborating. "The big one is coming soon."

Trump, speaking at a White House event, also for the first time clearly laid out what he said were four objectives.

They include destroying Iran's missile capabilities and "annihilating the navy."

"We've knocked out already ten ships. They're at the bottom of the sea," Trump said.

He said that the other goals were stopping Iran from obtaining a nuclear weapon and "ensuring the Iranian regime can't continue to arm, fund, and direct terrorist armies outside of their borders."

US and Israeli forces have so far struck hundreds of targets across Iran, including the Islamic republic's missiles, navy, and command-and-control sites.

Four US military members have been announced killed, and three fighter jets have been shot down -- officially in friendly fire.

Iran has fired missiles at Israel, at US bases around the region, and also at targets in regional Arab countries – Bahrain, Jordan, Kuwait, Qatar, and the United Arab Emirates – something that Trump called "the biggest surprise."

Trump, speaking at the White House award ceremony, said that the US could sustain the assault beyond the month-long timeframe he has publicly spoken about.

Some analysts have wondered if the US, even with the world's most powerful military, has enough ammunition to carry out such a long war against a determined foe.

"We're already substantially ahead of our time projections," Trump said at the White House, adding: "From the beginning, we projected four to five weeks, but we have capability to go far longer than that. We'll do it."

Defense Secretary Pete Hegseth also signaled Monday that deploying troops inside Iran had not been ruled out.

Asked if there were already boots on the ground, Hegseth told a news conference: "No, but we're not going to go into the exercise of what we will or will not do... We'll go as far as we need to go."