Iranian officials accused the United States of violating the ceasefire with the tanker strikes and hampering diplomatic efforts to end the conflict.

A parallel ceasefire in Lebanon was also under strain. Iran-backed Hezbollah launched missiles and drones at military bases in Israel in retaliation for a recent attack on Beirut and ongoing strikes in the south, where Lebanese authorities reported 11 people killed on Friday.

US Central Command said an F/A-18 Super Hornet used precision munitions on Friday against two ships in the Gulf of Oman -- gateway to the vital Strait of Hormuz -- to prevent them from continuing to Iran.

An Iranian military official told local media the country's navy had "responded to the violation of the ceasefire and to American terrorism with strikes" and "the clashes have now ceased."

The latest incident came after another flare-up overnight in the strait, control of which an adviser to Iran's supreme leader compared to having "an atomic bomb."

US Secretary of State Marco Rubio reiterated on Friday that it was "unacceptable" for Tehran to control the crucial oil conduit.

Speaking to reporters in Rome, Rubio said Washington was expecting Iran's response to its latest proposal later in the day and expressed hope it would be "a serious offer."

Trump, at the White House, later added: "I'm getting a letter supposedly tonight, so we'll see how that goes."

Washington has sent Iran, via Pakistani mediators, a proposal to extend the truce in the Gulf to allow for talks on a final settlement of the conflict launched 10 weeks ago with US-Israeli strikes on Iran.

Iran's foreign ministry spokesman Esmaeil Baqaei said Friday that the proposal was still "under review," according to the ISNA news agency.

- Oil slick -

Iran's UN envoy, Amir Saeed Irvani, accused the United States of violating the ceasefire with the attacks on the Iranian tankers, in a letter to the UN secretary-general and Security Council.

Qatar's prime minister, Sheikh Mohammed bin Abdulrahman Al Thani, met with US Vice President JD Vance in Washington on Friday and discussed the Pakistani-led efforts to broker a permanent peace.

Iran has repeatedly attacked sites in Qatar during the war, pointing to the wealthy emirate's role as host of a major US air base.

Satellite images have meanwhile shown that an oil slick is spreading off the coast of Iran's Kharg Island, a key oil export terminal for the Islamic republic.

It was not immediately clear what had caused the apparent spill, which was located off the island's west coast and appears to cover more than 20 square miles (52 square kilometres), according to global monitor Orbital EOS.

Kharg Island is at the heart of Iran's oil export industry, a lynchpin of the country's battered economy, and lies in the Gulf, north of the narrow Strait of Hormuz.

Following the start of the war on February 28, Iran largely closed the strait, throwing global markets into turmoil and driving up oil prices. The US later imposed its own blockade of Iranian ports in response.

On Sunday, Trump announced a US naval operation designed to reopen the strait to commercial shipping, only to abandon it on Tuesday in favour of a return to negotiations.

Saudi sources told AFP on Friday that the kingdom had refused permission for the US military to use its bases and airspace for the Hormuz operation, with one saying Riyadh "felt it would just escalate the situation and would not work."

- Lebanon front -

On the war's other front, Hezbollah said a salvo of missiles targeted a base south of the Israeli city of Nahariya on Friday in response to Israeli targeting of Beirut's southern suburbs and southern Lebanon.

Hours later, the Iran-backed group announced it had launched a swarm of drones at another base in northern Israel.

Israel has kept up its strikes on Hezbollah in spite of a ceasefire, and on Wednesday it carried out its first attack on Beirut's southern suburbs in a month, saying it killed a senior Hezbollah commander.

The Lebanese health ministry said Israeli strikes in the south killed 10 people on Friday, including two children and three women. Lebanon's civil defence said one of its members was killed.

The latest violence came as Lebanon and Israel, officially at war since 1948, were set to hold direct negotiations in Washington next week, which Hezbollah vehemently opposes.

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