President Trump faces new diplomatic tests as he prepares for weekend talks with Tehran amid doubts about the durability of his day-old cease-fire with Iran and the prospects for building it into a broader peace settlement.

A White House official said on Wednesday that Vice President JD Vance would lead a U.S. delegation to Pakistan for a meeting on Saturday with Iranian officials. Mr. Vance will be joined in the capital, Islamabad, by Mr. Trump’s special envoy Steve Witkoff and his son-in-law Jared Kushner as they work to bridge huge political differences, some dating back decades, under a two-week clock set by the cease-fire agreement.

But even as Trump officials finalized the Saturday meeting, fractures were already emerging in the limited cease-fire brokered by Pakistan on Tuesday night, just ahead of Mr. Trump’s deadline for a threatened “civilization”-ending attack on Iran.

Robert Malley, who served as President Joseph R. Biden Jr.’s special envoy for Iran, said the cease-fire was filled with ambiguities. The United States and Iran are already arguing over them, he said, and that will complicate the path forward.

“It’s hard to know not just where you go from here, but where you are to begin with,” he said. “The talks are starting on very weak grounds.”

In a statement on social media, Iran’s parliament speaker, Mohammad Bagher Ghalibaf, insisted that three clauses of what he said was a 10-point “agreed framework” between the U.S. and Iran had already been violated, including an end to Israeli attacks on Iran-backed Hezbollah fighters in Lebanon. The Trump administration says that was not part of the agreement.