Iran’s Islamic Revolutionary Guards Corps said that its naval forces had seized two cargo ships near the Strait of Hormuz on Wednesday, the Iranian news media reported, as Tehran renewed its efforts to exert control over the strategic waterway.

Hours after President Trump extended a cease-fire with Iran, the Revolutionary Guards’ Navy said that two vessels, the MSC Francesca and the Epaminondas, had tried to navigate through the contested strait “without the necessary permits,” according to a statement shared by the semiofficial Tasnim news agency. The statement said that the ships were being directed toward Iran’s coast, adding: “Disrupting the order and security of the Strait of Hormuz is our red line.”

Earlier on Wednesday, United Kingdom Maritime Trade Operations, a British Navy agency, said that two cargo vessels had come under attack in the area of the strait. One of them, a container ship sailing off the coast of Oman, was said to have been struck by a gunboat belonging to the Guards. The ship’s captain said that the gunboat gave no radio warning before firing on it, according to the agency, which said that no crew members were hurt but that the ship’s bridge had been heavily damaged.

The British agency said that a second unnamed outbound cargo ship had come under fire eight nautical miles off the coast of Iran. The captain of that vessel said crew members were unharmed, according to the agency, which did not name either vessel.

MSC, the global shipping company that owns the Francesca, did not immediately respond to requests for comment. The vessel was flying a Panamanian flag and bound for Sri Lanka.

Technomar Shipping, a Greek company that manages the Epaminondas, said in a statement that the ship had been attacked by a gunboat when it tried to pass through the strait, but would not confirm whether Iran had seized the vessel. The crew of the ship, which was sailing under a Liberian flag, were safe and accounted for, the company said.

Greece’s foreign minister, Giorgos Gerapetritis, confirmed that a Greek-owned, Liberian-flagged cargo ship had been attacked on Wednesday. In an interview with CNN, he said that the ship, which he declined to name, had sustained “extremely wide damage” but was not at risk of sinking. “All seafarers are in good health,” he added.

Daniel Mueller, a senior analyst at the maritime intelligence firm Ambrey, said that location data transmitted by the two seized ships on Wednesday afternoon suggested they were anchored about eight miles off the Iranian coast, which could indicate that they were under Iranian control.

He said the ships were intercepted as they attempted to exit the strait, suggesting that Iranian authorities had previously permitted them to enter — most likely the night before. Data shared by MarineTraffic, a website that tracks shipping, indicates that the ships were located near Dubai on Saturday, when they were traveling toward the strait.

Mr. Mueller, citing publicly available data and shipping industry information, said that a third cargo ship — the Euphoria — also appeared to have had a close brush with Iranian forces on Wednesday, but that details were murky.

That ship was sailing close to Larak Island, which Iran has established as a route for vessels it approves to navigate the strait, when it stopped multiple times and changed direction abruptly, according to tracking data. Mr. Mueller said that could indicate that the ship had received instructions from Iranian authorities.

“The Euphoria is now further south, not quite clear of the area in general, but they have been able to continue,” he said.

Earlier on Wednesday, several Iranian news media outlets reported that the Islamic Revolutionary Guards Corps had fired on the Euphoria and that it was stranded off the coast of Iran. By the afternoon, the vessel appeared to have passed through the strait, with tracking activity by Kpler, a maritime data firm, showing that it was in the Gulf of Oman. The condition of the ship was not known.

The two seizures showed how the Strait of Hormuz remains heavily contested by both the United States and Iran even after Mr. Trump renewed the cease-fire.

In announcing the extension of the truce, just before it had been set to expire, Mr. Trump said that the U.S. Navy would keep in place a blockade of Iranian ports — which Iranian officials have described as a violation of the truce. The U.S. Navy has turned back more than two dozen ships trying to enter or leave Iranian ports since it began the blockade more than a week ago, according to the U.S. military’s Central Command.

The Strait of Hormuz, normally a conduit for one-fifth of global oil and a significant share of gas supplies, remains a high-risk area, maritime security officials said, while the future of U.S.-Iran negotiations remains unclear. Since March 1, the day after the U.S.-Israeli strikes on Iran began, the British Navy’s maritime trade agency has recorded 34 security incidents in the strait and the surrounding waters of the Persian Gulf and the Gulf of Oman.

Multiple ships have been struck by unknown projectiles, causing hull damage or fires on board. Iran’s Revolutionary Guards have also threatened vessels, the agency said.

“The operating environment remains high-risk,” the agency said in a report on Tuesday. “The cease-fire remains fragile, and blockade-related enforcement activity continues.”

European military leaders were meeting on Wednesday in London to discuss how to reopen the critical shipping route. Britain and France were hosting military planners from more than 30 countries to “advance military plans to reopen the strait, as soon as conditions permit, following a sustainable cease-fire agreement,” according to a statement from Britain’s defense minister.

The meeting builds on an earlier gathering of officials from more than 50 countries to condemn the closure of the strait, and to call for a diplomatic solution.

Peter Eavis contributed reporting.