On Monday, Iran threatened repercussions. Ebrahim Zolfaghari, an Iranian military spokesman, said that if Iranian ports were threatened, “no port in the Persian Gulf and the Sea of Oman will be safe.”

European leaders, already frustrated by Mr. Trump’s military campaign in Iran, quickly distanced themselves from his proposed blockade. Prime Minister Keir Starmer of Britain said in a radio interview that the United Kingdom would not participate, while Spain’s defense minister said the maneuver “makes no sense.”

The U.S. military said that it would begin blocking ships “entering or departing Iranian ports and coastal areas” starting at 10 a.m. Eastern on Monday, while allowing other vessels to transit the strait on their way to or from non-Iranian ports.

A U.S. blockade of Iranian ports would be a significant change in Washington’s approach. The United States had previously been allowing Iranian oil tankers to traverse the strait to keep up global supplies, according to Scott Bessent, the Treasury secretary.

Iran has already choked off shipping through the critical waterway — through which about 20 percent of the world’s oil normally flows — since the war began in late February, allowing only its own ships and a few others to pass.

As markets opened Sunday evening, the price of Brent crude, the global benchmark for oil, rose more than 7 percent to around $102 a barrel. After Mr. Trump announced the cease-fire last week, Brent crude, which had climbed as high as $119 a barrel in late March, fell to $95 a barrel. Global oil prices have soared more than 50 percent during the conflict.

On Sunday, an adviser to Mojtaba Khamenei, the supreme leader, warned that Iran had “large, untouched levers” to counter any naval blockade. But other Iranian officials appeared to leave the door open for further diplomacy.

On Sunday night, Mr. Trump told reporters that the cease-fire was “holding well” and that Iran may yet return to the negotiating table. But he claimed not to be concerned either way.

“I don’t care if they come back or not,” he said. “If they don’t come back, I’m fine.”

Here’s what else we’re covering:

• Negotiations in Islamabad: Talks ended with the U.S. delegation saying that Iran’s refusal to give up its nuclear program was a sticking point, while Tehran accused Washington of a “maximalism approach.” The key differences between the two sides center on the fate of nearly 900 pounds of highly enriched uranium, frozen Iranian revenues held abroad, and control of the strait.

• Strait of Hormuz: France and Britain will organize a conference in the coming days to continue discussing a multinational mission to secure passage for ships through the strait, President Emmanuel Macron of France said on Monday. European countries have refused to meet Mr. Trump’s demand that they send warships now, and are discussing instead what to do after the war ends. But their options are limited.

• Israel and Lebanon: The Israeli military says its forces have encircled and raided the Lebanese town of Bint Jbeil, one of the largest communities near the border between the two countries. Israel’s attacks there have become a sticking point in the cease-fire negotiations, as Iran has demanded the truce extend to Lebanon. According to the Lebanese government, more than 2,000 people have been killed in the country since Hezbollah, the Iran-backed militia, fired on northern Israel in early March, prompting a widening Israeli ground invasion. The Israeli and Lebanese ambassadors to the United States are expected to meet in Washington this week for rare direct talks.

• Death tolls: The Human Rights Activists News Agency said at least 1,701 civilians, including 254 children, had been killed in Iran as of Wednesday. Lebanon’s health ministry on Saturday said that 2,020 people had been killed in the latest fighting between Israel and Hezbollah, including 357 in a wave of Israeli strikes on Wednesday. In attacks attributed to Iran, at least 32 people have been killed in Gulf nations. At least 22 people had been killed in Israel as of Sunday, as well as 12 Israeli soldiers fighting in Lebanon. The American death toll stands at 13 service members.