A parallel ceasefire in Lebanon was also under strain on Friday, as Iran-backed Hezbollah launched missiles at a military base in Israel in response to a Beirut strike that killed a top commander and other attacks in the south.

The US Central Command said an F/A-18 Super Hornet used precision munitions 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", adding that after the "exchanges of fire, 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 strait, adding that Washington was expecting Iran's response to its latest proposal later in the day.

"I hope it's a serious offer, I really do," he told reporters during a trip to Rome.

Washington has sent Iran, via Pakistani mediators, a proposal to extend the truce in the Gulf to allow 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 on Friday that the proposal was still "under review, and once a final decision is reached, it will certainly be announced", according to the ISNA news agency.

- 'They trifled with us' -

The night before, US Central Command said Iran had launched missiles, drones and small boats at three US warships transiting the Strait of Hormuz, but that none were hit and US forces had retaliated against land bases in Iran.

Iran's central military command, Khatam al-Anbiya, countered that the clash had erupted when US vessels targeted an Iranian tanker heading towards the strait, and accused its foe of hitting civilian areas.

Asked in Washington on Thursday if the truce was still in effect after the clash, US President Donald Trump said: "Yeah, it is. They trifled with us today. We blew them away."

Iran accused regional US allies of cooperating in the strikes, without naming them, though the United Arab Emirates said it had been forced to intercept a volley of Iranian drones and missiles that wounded three people.

Following the start of the war on February 28, Iran largely closed the Strait of Hormuz, 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.

On Friday, Saudi sources told AFP 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".

Mohammad Mokhber, an adviser to Iran's supreme leader, said the Islamic republic had long "neglected" its privileged position along the strait, calling it "an opportunity as precious as an atomic bomb".

"Indeed, having in one's hands a position that allows you to influence the global economy with a single decision is a major opportunity," he added, vowing not to relinquish it.

This week Tehran created an authority to approve transit through the strait and to collect tolls from vessels, according to leading shipping industry journal Lloyd's List.

- Lebanon front -

On the war's other front, in Lebanon, Hezbollah said it fired missiles at a military base in Israel in retaliation for its "targeting of Beirut's southern suburbs and the attacks that affected villages and civilians" in the south.

Air raid sirens sounded in several cities in northern Israel, according to the Israeli military, and Hezbollah also claimed several attacks on its troops in southern Lebanon.

Israel has kept up its strikes on the Iran-backed group 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 fresh Israeli strikes on Friday in the southern Tyre district killed four people, while the state-run National News Agency reported a series of attacks across the south.

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.

burs-smw/dcp