Washington DC - President Donald Trump said Thursday that Iran had at most 15 days to make a deal on concerns starting with its nuclear program, suggesting the US would attack if it did not.

"We're either going to get a deal, or it's going to be unfortunate for them," Trump told reporters on Air Force One.

Asked about his timeline, Trump said, "I would think that would be enough time – 10, 15 days, pretty much maximum."

Trump earlier Thursday had suggested 10 days for Iran's clerical state to reach an agreement.

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"It's proven to be over the years not easy to make a meaningful deal with Iran. We have to make a meaningful deal; otherwise, bad things happen," Trump told the inaugural meeting of the "Board of Peace," his initiative to secure stability in Gaza.

He warned at the event in Washington that the US "may have to take it a step further" without any agreement, adding, "You're going to be finding out over the next probably 10 days."

Trump's friend and roving envoy Steve Witkoff and the president's son-in-law Jared Kushner met Tuesday indirectly in Geneva with Iran's top diplomat, who said that there was progress.

The talks come a month after Iranian authorities killed thousands of people as they crushed one of the biggest threats yet to the Islamic republic established in the 1979 revolution that toppled the pro-Western shah.

Secretary of State Marco Rubio is expected to hold talks on February 28 in Israel with Prime Minister Benjamin Netanyahu, who has long advocated tough action against Tehran and ordered a bombing campaign last year.