US President Donald Trump has indicated that the military conflict with Iran could extend beyond an initially suggested timeframe, saying it might last about four weeks—or even longer if necessary.

In an interview, Trump described the operation against Iran as a complex campaign and said planners had anticipated it would take around four weeks to achieve key objectives. He added that the US is prepared to sustain the effort beyond that period if needed, and he wouldn’t get “bored” if the war dragged on.

Trump’s remarks come amid an intensifying conflict that has already seen significant casualties on both sides. US military strikes have targeted Iranian military and political leadership, while Tehran has responded with missile and drone attacks against US, Israeli, and Gulf targets. Three US service members were confirmed killed, and Trump said more casualties were likely before the conflict ends.

He also 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 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."

"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."