Washington DC - US President Donald Trump called Thursday on Israel not to attack Iran, saying a deal on its nuclear program remained close despite Tehran defiantly vowing to increase its output of enriched uranium ahead of new talks.

Trump acknowledged that an Israeli strike on Iran "could very well happen," although he stopped short of calling a strike imminent, and said the risk of "massive conflict" led the US to draw down staff in the region.

"We are fairly close to a pretty good agreement," Trump told reporters.

Asked about his discussions with Israeli Prime Minister Benjamin Netanyahu, Trump said: "I don't want them going in, because I think it would blow it."

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Trump quickly added, "Might help it actually, but it also could blow it."

Trump's Middle East pointman Steve Witkoff is set to hold a sixth round of talks with Iran on Sunday in Oman, which has mediated.

Trump again described himself as a man of peace.

"I'd love to avoid the conflict. Iran's going to have to negotiate a little bit tougher – meaning they're going to have to give us some things that they're not willing to give us right now," he said.

Iran has ramped up rhetorical pressure before the talks, including with a threat to strike American bases in the region if the negotiations break down and conflict erupts.