Strikes on commercial ships in the Strait of Hormuz presented another test of the fragile cease-fire between the United States and Iran, as President Trump flew on Tuesday to a NATO summit where discussions about the war were expected.

Iranian missiles hit two ships in the strait, but there were no casualties, a U.S. official said late Monday Eastern time.

According to a notice issued early on Tuesday by United Kingdom Maritime Trade Operations, a monitoring center led by the British navy, the crew of a tanker off the coast of Oman reported a strike by an unidentified projectile, which caused a fire on the vessel.

The report did not identify the tanker or its cargo, and said that no casualties or environmental effects were reported. The tanker was near the eastern mouth of the strait when it was hit, according to the report.

There was no immediate public comment from the authorities in Iran, where a dayslong program of funeral ceremonies is underway for Ayatollah Ali Khamenei, the supreme leader who was killed on the first day of the war. Negotiations between Iran and the United States have been paused until after the funeral.

Mr. Trump, who was on his way to Turkey for the NATO summit, also did not immediately comment on the reports of strikes on ships in the strait. He has criticized NATO’s members for not supporting the United States in the war against Iran.

The strait, normally the conduit for a fifth of the world’s oil, was effectively blockaded by Tehran after the United States and Israel started the war with attacks on Iran in late February. The U.S. Navy also imposed its own blockade of Iranian ports. Economies around the world were hit by the energy supply crunch and rise in prices that followed.

Traffic around the strait has picked up since June 20, around when the preliminary cease-fire agreement between the United States and Iran went into effect. That agreement has already been challenged by sporadic outbreaks of fighting. The latest strikes were reported nearly two weeks after another bout of attacks on ships by Iran, which prompted U.S. retaliation against Iranian military infrastructure.

From Friday through Sunday, 108 ships passed through the Strait of Hormuz, according to Kpler, a maritime data company. That was 21 fewer than the three prior days. Before the war, more than 100 ships a day routinely passed through the strait.

Iran has said it expects ships to pass through the strait along its coast, not on the opposite side near Oman. The traditional route through the middle of the strait is considered dangerous because of the risk of mines laid by Iran’s military.

Many ships switch off their transponders before navigating the strait, making it hard to identify their precise routes and giving an incomplete view of traffic volumes.

The price of Brent crude oil, the global benchmark, rose more than 1 percent on Tuesday, to $73 a barrel. As energy exports from the Persian Gulf have recovered, however tentatively, the price of oil has fallen back to near prewar levels. The price of Brent had spiked as high as $118 a barrel during the worst of the fighting.

The effects of the energy shock linger, however, especially in products derived from oil. The average price of a gallon of gasoline in the United States was $3.79 on Tuesday, according to the AAA motor club, about 27 percent higher than before the war.