Trump, talking to reporters as he left the White House on a trip to Texas, said Iran was still unwilling to forswear nuclear weapons as demanded by the United States.
He spoke a day after negotiations between US envoys Steve Witkoff and Jared Kushner and Iranian officials in Geneva ended with no deal.
A massive US military presence is in the region waiting on Trump's order.
Asked about the potential for use of force, Trump said the United States has the greatest military in the world.
"I'd love not to use it but sometimes you have to," he said.
Trump said more discussions on Iran would take place later in the day.
He did not specify with whom, but top US defence officials were at the White House on Thursday for talks.
"We want no nuclear weapons by Iran and they're not saying those golden words," Trump said.
The president planned events in Corpus Christi, Texas, later on Friday and then was to fly on to Palm Beach, Florida, for the weekend at his Mar-a-Lago club.
Oman, which has been acting as a mediator between the United States and Iran, sent its foreign minister to Washington on Friday for discussions on the issue with US Vice President JD Vance, according to a source familiar with the matter.
A source briefed on the internal White House deliberations told Reuters that Trump, who launched a bombing raid against Iranian nuclear sites last June, is "very clear-eyed on all the options before him".
There is a recognition internally that taking on Iran would be more difficult than the US capture of Venezuelan leader Nicolas Maduro, and there was also internal pessimism as to whether negotiations will bear fruit, the source said.
"Nobody is super optimistic about the negotiations," the source said.
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