“Based on the fact that the Government of Iran is seriously fractured, not unexpectedly so and, upon the request of Field Marshal Asim Munir, and Prime Minister Shehbaz Sharif, of Pakistan, we have been asked to hold our Attack on the Country of Iran until such time as their leaders and representatives can come up with a unified proposal,” Mr. Trump wrote.

“I have therefore directed our Military to continue the Blockade and, in all other respects, remain ready and able, and will therefore extend the Ceasefire until such time as their proposal is submitted, and discussions are concluded, one way or the other.”

The tone of Mr. Trump’s message was starkly different from earlier in the day, when he had threatened a military attack if Iran did not agree to U.S. demands. “I expect to be bombing,” Mr. Trump said on CNBC on Tuesday morning. “The military is raring to go.”

The back-and-forth is part of a by-now familiar pattern by Mr. Trump of threatening and then pulling back, leaving the fate of the negotiations — and the war — in flux. It also marks yet another hurdle in the Trump administration’s push to secure an agreement that would curb Iran’s nuclear program, and it comes as the Pentagon has been reviewing military options should Mr. Trump conclude that Tehran is not negotiating in good faith.

Still, a U.S. official with direct knowledge of the situation said a return to bombing was not imminent even as the Pentagon continued to game out options. The United States remains well-positioned to launch another wave of strikes, having kept its substantial military presence in the Middle East.

The official also cautioned that Mr. Vance’s trip to Pakistan could be back on at a moment’s notice with the president’s approval. U.S. officials also are looking for a clear sign that Iran’s negotiators have been fully empowered to reach an agreement.

The United States recently transmitted a written proposal to the Iranians intended to establish base-line points of agreement that could frame more detailed negotiations. The document covers a broad range of issues, but the core sticking points are the same ones that have bedeviled Western negotiators for more than a decade: the scope of Iran’s uranium enrichment program and the fate of its stockpile of enriched uranium.

It’s unclear what exactly the United States has proposed or what the president would be willing to accept. On enrichment, the American position could range from demanding that Iran abandon enrichment entirely to allowing a limited civilian program under strict oversight by the International Atomic Energy Agency, paired with the closure of Iran’s underground nuclear facilities.

One of the ideas discussed during negotiations last year was a multinational consortium working with Iran to enrich uranium for civilian uses; potential locations included an island in the Persian Gulf. Regarding the stockpile, negotiators are weighing options including whether Iran should surrender its enriched uranium directly to the United States or transfer it to a third country.

Also on the table is what the United States might offer in return. Iran has hundreds of billions of dollars in assets frozen under American sanctions as part of Mr. Trump’s maximum-pressure campaign, and administration officials are debating whether releasing some of those funds could be part of a final deal. Officials have also discussed whether the United States and gulf partners such as Saudi Arabia and the United Arab Emirates might offer broader economic integration to Iran.

Mr. Trump has been adamant in private conversations that his deal must be better than the one struck by President Barack Obama in 2015. Knowing that, Iran hawks close to the president have repeatedly invoked Mr. Obama’s deal as a tactic to keep him from agreeing to what they view as dangerous concessions.

Any American position on enrichment will have to contend with Iran’s longstanding argument, rooted in its accession to the Nuclear Nonproliferation Treaty, that the pact guarantees signatories the right to enrich uranium for peaceful purposes.

The negotiations have been run by a tight circle inside the administration. Mr. Vance, along with Jared Kushner and the envoy Steve Witkoff, have served as the principal American interlocutors, with Mr. Vance and Mr. Kushner, the president’s son-in-law, doing most of the talking. On the Pakistani side, Field Marshal Syed Asim Munir has emerged as the critical channel to the Iranians.

Back in Washington, Situation Room meetings on Iran have typically included Mr. Trump; the White House chief of staff, Susie Wiles; Secretary of State Marco Rubio; the director of national intelligence, John Ratcliffe; and Mr. Vance. Defense Secretary Pete Hegseth and Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, also participate.

Treasury Secretary Scott Bessent has become increasingly involved as the economic dimensions of a potential deal have come into sharper focus, as has the energy secretary, Chris Wright, though Mr. Trump has vented frustrations with Mr. Wright in recent days after the energy secretary told CNN that gas prices might not drop below $3 until next year. Mr. Trump told The Hill that Mr. Wright was “totally wrong.”

The pause in talks caps a turbulent few days of public messaging from Mr. Trump, whose statements have at times appeared at odds with the state of the negotiations.

In a telephone interview with CBS News on Friday, Mr. Trump declared that Iran had “agreed to everything” and described a joint operation to remove Iranian nuclear material. “Our people, together with the Iranians, are going to work together to go get it. And then we’ll take it to the United States,” he said. Iranian officials quickly disputed the characterization.

Then on Sunday, Mr. Trump wrote on Truth Social that Iran had violated the cease-fire by firing on commercial vessels in the Strait of Hormuz, including a French ship and a British freighter. That same day, U.S. military forces seized an Iranian-flagged ship, the Touska, that Mr. Trump said tried to evade its blockade on the country’s ports.

The president said his representatives would arrive in Islamabad the following evening for negotiations, and he warned that if Iran rejected what he called “a very fair and reasonable DEAL,” the United States would “knock out every single Power Plant, and every single Bridge, in Iran.”

“NO MORE MR. NICE GUY!” he wrote.