President Trump, in a sudden reversal, announced on Tuesday evening that the United States was pausing “for a short period of time” efforts to help guide ships through the Strait of Hormuz, which Iran has effectively blocked to most commercial ships for weeks.

The president’s announcement came one day after the effort began. He said in a social media post that the change was “based on the request of Pakistan and other countries,” as well as the “tremendous military success” and “great progress” toward an agreement. Pakistan has hosted cease-fire talks between American and Iranian negotiators.

Mr. Trump said, however, that a recently announced U.S. blockade would “remain in full force and effect” in the narrow waterway and that the pause was “to see whether or not the agreement can be finalized and signed.”

Earlier on Tuesday, Mr. Trump and top American officials had said that a shaky cease-fire with Iran was holding despite new attacks by both sides after the American military launched an effort to reopen the strait.

After the United States and Israel began strikes in Iran on Feb. 28, Tehran claimed control of the vital oil shipping route off Iran’s southern coast, through which about 20 percent of the world’s oil and liquid natural gas had passed. In April, the United States announced its own blockade of the strait. Mr. Trump threatened to wipe out a “whole civilization” if Iran did not open the strait. Then the United States said it would “guide” trapped vessels through the waters there.

Only two ships were known to have passed through the waterway on Monday. None appear to have made the trip on Tuesday.

Earlier on Tuesday, Defense Secretary Pete Hegseth described the U.S. operation to open the strait, which began on Monday, as defensive and temporary. “We’re not looking for a fight,” he told reporters at the Pentagon. He and Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, said the cease-fire remained in place, despite recent attacks.

Mr. Hegseth added the United States was still working to free trapped vessels. “We’re ensuring that we have control of that strait, which we do,” he said. Two commercial vessels crossed the strait under American military protection on Monday, he said, traveling through a sea lane cleared of mines by naval robots.

Iran’s state broadcaster dismissed the U.S. effort as a failure and said Iranian control over the strait had “intensified.”

About 1,600 ships remain stranded in dangerous conditions at the strait, and overall ship traffic is a trickle compared with before the war, when around 130 vessels a day made the passage. Oil and gas prices have spiked as a result, driving up costs for consumers and businesses around the world.

Mr. Trump’s announcement late Tuesday came after the United Arab Emirates said that for the second day in a row, it had come under attack from Iranian missiles and drones, which its air defense systems were intercepting. Iran denied that it was behind the attacks. It was not immediately clear if the strikes had caused any casualties or damage.

Earlier, speaking to reporters in the Oval Office on Tuesday, Mr. Trump would not specify what Iranian actions would violate the cease-fire. The Iranians “know what to do,” he said, and “they know what not to do, more importantly.”

On Monday, U.S. Central Command said American forces had shot down Iranian cruise missiles and drones aimed at U.S. ships and commercial vessels the Navy was guiding through the Strait of Hormuz. The U.S. military said it had also destroyed six Iranian speedboats that had threatened the vessels.

Iranian state media reported that American forces had attacked two small boats carrying cargo from Oman to Iran, killing five civilians on Monday.

General Caine said on Tuesday that Iran had attacked U.S. forces more than 10 times since the cease-fire took effect in early April, but that the attacks had been “all below the threshold of restarting major combat operations at this point.”

Secretary of State Marco Rubio said at a news briefing at the White House on Tuesday that the U.S. effort to reopen the strait was aimed at freeing more than 20,000 sailors from dozens of countries who have been trapped in the region since the war began. At least 10 civilian sailors have been killed in that time, he noted, using a figure that had also been cited by the United Nations.

Mr. Rubio said that the American military attack on Iran had ended, echoing arguments Mr. Trump made to Congress last week when he contended that he did not need to seek legislative approval for the war. Democrats and some legal experts have said that a Vietnam-era law requires Mr. Trump to seek congressional authorization to continue the military operation.

“The operation is over,” Mr. Rubio said. “Epic Fury, as the president notified Congress, we’re done with that stage of it. We’re now on to this project of freedom.” Operation Epic Fury is the Trump administration’s name for the war on Iran. Project Freedom is the name for the effort to reopen the Strait of Hormuz, which Mr. Rubio said was defensive, not offensive.

Mr. Trump predicted that reopening the strait would eventually lead to lower gas prices, which averaged $4.48 a gallon in the United States on Tuesday.

Iran’s navy, Mr. Trump said, has been so thoroughly destroyed that it is using “little boats with pea shooters.” But U.S. intelligence agencies have acknowledged that Iran still has thousands of missiles and drones, as well as armed speedboats, that can harass commercial ships.

General Caine described what he said was a defensive umbrella over the strait, comprising armed surveillance drones, Army attack helicopters, Air Force fighter jets, Navy warships — including two aircraft carriers — and 15,000 military personnel in the region.

Despite the formidable array of American firepower, most shipping companies may not feel safe enough to send their vessels through the strait, said Jack Kennedy, the head of Middle East and North Africa country risk at S & P Global Market Intelligence.

“Iran still retains capacity to deter most transit through the strait with its asymmetric military capabilities,” Mr. Kennedy said.

At the center of the conflict is the fate of Iran’s nuclear program, which has been the subject of fitful negotiations between Iranian and American officials. The Trump administration says it wants to ensure that Iran can never obtain a nuclear weapon. But more than two months after starting the war, the administration has outlined no clear path to achieve that goal.

“One way or another, hopefully, Iran chooses a deal that they give up those ambitions, give up those capabilities,” Mr. Hegseth said on Tuesday.

Pakistan said on Tuesday that a lasting peace agreement was needed both for the region and for the global economy.

“Obviously, it takes two to tango,” the Pakistani foreign minister, Ishaq Dar, said in a televised address. In a social media post, he also condemned the attacks on the United Arab Emirates and expressed his “full solidarity” with the country.

A parallel cease-fire in Lebanon, where Israeli forces have been battling Hezbollah, the Iranian-backed militia, has also come under growing strain. On Tuesday, Israeli airstrikes and artillery fire targeted several southern Lebanese towns, according to Lebanon’s state-run news agency. Hezbollah said it had launched drone attacks against Israeli troops in southern Lebanon.

Israeli and Lebanese government officials announced the cease-fire last month, but Hezbollah was not part of the negotiations, which the Trump administration hosted. Since then, Israel and Hezbollah have repeatedly accused each other of violating the truce.

Reporting was contributed by Helene Cooper, Euan Ward, Anushka Patil and Michael Levenson.