US President Donald Trump urged Iran Friday to make a deal or face "even more brutal" attacks, while keeping the door open for negotiations after Israel's deadly strikes on Tehran's nuclear facilities.

Trump appeared to be sitting on the fence, a day after having publicly told Israeli Prime Minister Benjamin Netanyahu to hold off striking Iran only to see the key US ally go ahead anyway.

The operation killed senior figures — among them the armed forces chief and top nuclear scientists — and Iran has called Israel's wave of strikes a "declaration of war."

"There has already been great death and destruction, but there is still time to make this slaughter, with the next already planned attacks being even more brutal, come to an end," Trump said on his Truth Social platform. 

"Iran must make a deal, before there is nothing left... JUST DO IT, BEFORE IT IS TOO LATE," he said. 

Trump said that he "gave Iran chance after chance to make a deal."

He added that Israel — which Trump has closely aligned with since his return to the White House — has a lot of weapons thanks to the United States and "they know how to use it."

Trump was attending a National Security Council meeting Friday in the White House Situation Room.

'It's been excellent'

But Trump later appeared to be making a political calculation about how best to respond to Israel's attack.

Less than a day earlier Trump had called on Israel to hold off an attack to make room for diplomacy, only for the key US ally to hit Iran as Trump was holding a picnic at the White House for members of Congress.

By Friday morning, however, Trump was calling the Israeli offensive "excellent" during a round of phone calls with US media.

"I think it's been excellent," ABC News quoted him as saying. "And there's more to come. A lot more."

Trump also gave mixed signals about the extent of US involvement.

Secretary of State Marco Rubio had said Thursday that the United States was "not involved" in the strikes and warned Iran not to retaliate against any US forces in the region.

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.

His boasts about the "finest" US equipment that Israel had used -- a day before a huge parade in Washington on Trump's 79th birthday featuring US aircraft and tanks — also muddied the waters.

Trump earlier told Fox News he had been made aware of the Israeli strikes before they happened, and stressed that Tehran "cannot have a nuclear bomb."

But at the same time Trump said Iran could have a second chance to negotiate.

"They missed the opportunity to make a deal. Now, they may have another opportunity. We'll see," Trump told NBC.

Trump also indicated that the Iranians were "calling me to speak" after the attacks to suggest they wanted to make a deal, without offering specifics.

During Trump's first term, he pulled the United States out of a landmark agreement to relieve sanctions on Iran in return for curbs on its nuclear program.

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, they have foundered in recent days.