President Trump acknowledged on Thursday that there was a significant risk that Israel could soon attack Iran, but he said that “I don’t want them going in” while some progress was still possible on an agreement to shut down Iran’s easiest pathway to building a nuclear weapon.

Mr. Trump said that an attack was likely to destroy the chances for a diplomatic solution. “I think it would blow it,” he said, before arguing the other side of the equation, saying it “might help it actually, but it also could blow it.”

His statement came just hours after the administration announced that Steve Witkoff, Mr. Trump’s special envoy to the Middle East, planned to meet the Iranian foreign minister in Oman on Sunday for another negotiating session.

The somewhat contradictory signals from the White House came amid growing signs that Israel was preparing for a strike of unknown proportions against Iranian sites. Mr. Trump refused to give details of a conversation he held on Monday with Prime Minister Benjamin Netanyahu, or to say whether, in that conversation, he had offered any kind of go-ahead to the Israeli leader to proceed with a strike. In recent weeks, Mr. Trump has said that he has urged Mr. Netanyahu to hold off on any military action while negotiations were underway.

Nonetheless, when asked on Thursday if an attack could happen very soon, he said: “I don’t want to say imminent, but it looks like it’s something that could very well happen.” Several hours later, in a social media post, he wrote that the administration was “committed to a Diplomatic Resolution of the Iran Nuclear issue.”

Mr. Trump’s statements left American and European intelligence officials searching for any signs that Israel was getting ready, including evidence that it was massing the aircraft and missiles that would be needed for any sustained assault on Iran’s remaining air defenses, its missile stores and its nuclear sites.