The Iranian foreign minister, Abbas Araghchi, arrived in Oman on Saturday for talks on the future of the Strait of Hormuz. The visit came a day after U.S. officials said they expected Iran to publicly declare an end to attacks on commercial ships in the waterway.

Though a cease-fire was signed by the two countries last month, President Trump and Iran’s supreme leader have continued to trade threats. In a written message on Saturday, Ayatollah Mojtaba Khamenei vowed revenge for the killing of his father and predecessor, Ayatollah Ali Khamenei, saying it was the “demand of our nation.”

The rhetoric furthered the uncertainty hanging over the fragile truce. While the Trump administration had said the cease-fire would fully lift Iran’s blockade of the Strait of Hormuz, a vital waterway for global oil and gas shipments, that has yet to be borne out.

Instead, Iranian forces have fired on ships traveling on routes they deem unacceptable, prompting the U.S. military to retaliate with attacks on Iranian military sites. The United States also revoked a sanctions waiver that had temporarily permitted the sale of Iranian oil.

Some of the ships were traveling along a route close to Oman’s coastline guided and protected by the United States. Iran has insisted that its waters are the only permissible route through the strait, essentially bringing marine traffic under its control.

After days of clashes over the strait, U.S. officials said Friday that they expected Iran to issue a public statement in the coming days acknowledging that all channels through the Strait of Hormuz were open, and that Iranian forces would cease shooting at passing ships.

The officials, who spoke to reporters on the condition that they not be identified, said that if Iran did not issue the statement, and stick with it, “we’re not going to have a good outcome for them.”

The Iranian government did not immediately comment. In a statement on Friday night, Esmaeil Baghaei, the Iranian foreign ministry spokesman, said Mr. Araghchi’s visit to Oman would include a meeting with Badr Albusaidi, the Omani foreign minister, and discussion of facilitating “safe navigation through the Strait of Hormuz.”

So far, threats by President Trump and bombardment by American warplanes have not succeeded in forcing Iran to relax its grip on the strait. Instead, both countries are now engaged in a simmering standoff — neither all-out war nor peace — with Mr. Trump saying on Friday that June’s cease-fire agreement was “over.”

Later, Mr. Trump added a threat on social media, vowing that “1000 Missiles are Locked and Loaded and aimed at the Islamic Republic of Iran” should the country make good on its threats to assassinate him or try to do so. U.S. law enforcement says Iran has devised several plots to kill him, as well as other American government officials, over the years.

In a seeming retort, Iran’s supreme leader, Ayatollah Khamenei, said that the country would seek revenge for his father. “We pledge that we will avenge your pure blood and the blood of all those martyred,” he wrote in a message published in Iranian state media on Saturday.

It is his first statement since a weeklong funeral for the elder Ayatollah Khamenei, culminating in his burial in Mashhad on Thursday. Millions turned out for processions and ceremonies across several cities in Iran and Iraq.

The new supreme leader was absent from the funeral and has been neither seen nor heard in public since being appointed to succeed his father’s authoritarian rule in early March, though he has occasionally released written statements. He is believed to have been wounded on the first day of the war, according to Iranian and Israeli officials.

The latest round of fighting began this week after Iran fired on three ships traversing the strait, U.S. officials said, with some of those vessels linked to the Persian Gulf countries of Qatar and Saudi Arabia, both U.S. allies.

U.S. forces retaliated with two days of heavy bombardment, attacking about 170 Iranian military targets, according to the U.S. military. On Friday, a spokesman for the Iranian health ministry said on social media that at least 17 people were killed in the attacks. Iran’s state television identified at least eight of the dead as soldiers.

Iran fired waves of ballistic missiles and drones at Kuwait, Bahrain and, for the first time since the beginning of the truce, Jordan.

The battle of wills in the Persian Gulf has reinforced skepticism that Mr. Trump can reach a broader pact with Iran to rein in its nuclear program, a stated aim of the U.S.-Israeli war on Iran that began in late February. Under the June cease-fire agreement, the two countries were supposed to negotiate a more comprehensive agreement in 60 days — a time frame that appears increasingly remote.

Oman has circulated its own proposal for jointly administering the Strait of Hormuz alongside Iran, including the potential imposition of service fees on transiting ships. The plans would be a significant change from the prewar status in the strait, when boats generally passed freely.

Leily Nikounazar and David E. Sanger contributed reporting.