The White House said late Thursday that Vice President JD Vance was delaying his trip to Switzerland to negotiate with Iran, raising uncertainty over the next phase of discussions to end the conflict.

The announcement from the vice president’s office came on the same day that Mr. Vance held a news briefing at the White House defending the preliminary deal to end the war. Mr. Vance said during the briefing that he did not know whether he would still travel to Switzerland on Friday for the negotiations, where he was initially expected to help oversee a signing ceremony for the deal.

It is unclear when Mr. Vance might reschedule his trip.

Mr. Vance said earlier Thursday that his “plan is to go to Switzerland” but he did not know “exactly when.” “We think these technical negotiations are going to start sometime this weekend — that’s still the plan — but that could change.”

The plans appeared to change.

Days after administration officials said earlier this week that they would hold a signing ceremony for the deal on Friday, Iran’s Foreign Ministry said that Iran’s president had “digitally” signed the memorandum of understanding between the United States and Iran and that the signing ceremony in Switzerland was off, according to Iranian state media.

President Trump then signed the document for a second time on Wednesday night — after first signing it digitally on Sunday — at the Palace of Versailles before departing France, where he attended the Group of 7 summit. Mr. Trump returned to the White House early Thursday morning.

On Thursday evening, the White House confirmed that Mr. Vance’s trip was off, for now.

“The plans for the upcoming technical talks have not been finalized, and the U.S. delegation has been prepared to depart at the first available opportunity,” according to the White House statement. “But the logistics of these negotiations have never been simple or predictable.”

The White House is looking forward “to beginning technical talks as soon as possible,” according to the statement.

Mr. Vance said on Thursday that he planned to lead the U.S. negotiating team to work out a final agreement with Iran. On Thursday, he defended the deal by relying in part on a string of aspirational, vague and misleading claims amid mounting criticism that the deal would reward and embolden Iran without ensuring that the United States achieved the main objectives laid out by Mr. Trump at the start of the war.

Erica L. Green contributed reporting.