But Trump also kept the door open for negotiations on Tehran's nuclear program, as the president who boasted he had "no wars" in his first term tries to avoid getting dragged into one in his second.
After Iran launched barrages of ballistic missiles on Friday, the United States was "assisting in shooting down missiles targeting Israel," two US officials told AFP, without giving detail on the extent of Washington's role.
Trump attended a National Security Council meeting in the White House Situation Room as his team worked on the crisis. An aide posted a black and white picture of a grim-faced, purse-lipped Trump striding through the West Wing.
Israel struck Iran just hours after Trump publicly urged Netanyahu to hold off on an attack, with the first missiles landing as the US president hosted a picnic for lawmakers on the White House South Lawn.
'Excellent'
Trump said that he "gave Iran chance after chance to make a deal."
"I think it's been excellent," ABC News quoted him as saying about Israel's offensive. "And there's more to come. A lot more."
Trump also gave mixed signals about the extent of US involvement.
Trump, however, said on Truth Social on Friday that Israel had acted because a 60-day deadline that he had set for Iran had run out, implying that the two acted in concert.
Trump earlier told Fox News he had been aware of the Israeli strikes before they happened, and stressed that Tehran "cannot have a nuclear bomb."
The United States and Iran have had several rounds of talks since Trump returned to the White House, but after initially striking an optimistic tone, the discussions have foundered in recent days.
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