DUBAI/WASHINGTON, May 4 (Reuters) - Suspected Iranian strikes hit several ships in the Strait of Hormuz on Monday and set a UAE oil port ablaze, as President Donald Trump's attempt to use the U.S. Navy to free up shipping provoked the biggest escalation of the Iran war since a ceasefire was declared four weeks ago.

Trump's new mission "Project Freedom", which he announced on social media overnight, was the first apparent attempt to make use of naval power to unblock the world's most important energy shipping route.

But at least in the initial hours on Monday, the gamble appeared to have backfired, bringing no surge of merchant shipping through the strait while provoking a show of force from Iran, which had long threatened to respond to any escalation with new attacks on its neighbours.

The U.S. military said two U.S. merchant ships had made it through the strait, without saying when. Iran denied any such crossings had taken place.

The commander of U.S. forces in the region said his fleet had destroyed six small Iranian boats, which Iran also denied. Admiral Brad Cooper said he "strongly advised" Iranian forces to keep clear of U.S. military assets carrying out the mission.

Iranian authorities, for their part, released a map of what they said was an expanded sea area now under their control, which went far beyond the strait to include swathes of international waters, including long stretches of the United Arab Emirates' coastline on either side of the strait.

South Korea reported one of its merchant ships had been hit by an explosion and fire inside the strait. The British maritime security agency UKMTO reported two ships had been hit off the coast of the UAE, and the Emirati oil company ADNOC said one of its empty oil tankers was hit by Iranian drones while trying to cross.

After reported drone and missile attacks inside the UAE throughout the day, including one that caused a fire at an important oil port, the UAE said Iranian attacks marked a serious escalation and it reserved the right to respond.

STRAIT STILL BLOCKED

Trump has struggled to find a solution to the disruption of international energy supplies caused by Iran's blockade of the strait, which carried a fifth of global oil and liquefied natural gas before the war.

In the two months since Trump launched an air war against Iran alongside Israel, Tehran has largely blocked the strait to ships apart from its own. Since last month, the United States has imposed its own blockade of ships leaving and entering Iranian ports.

The warring sides issued contradictory statements on Monday about the initial impact of the new U.S. mission, and Reuters could not independently verify the full situation there.

But there was no immediate sign that large numbers of merchant ships were making new attempts to cross, and major shipping companies said they were likely to wait for an agreed end to hostilities before trying to sail through.

REVOLUTIONARY GUARDS SAY NO TRANSITS TOOK PLACE

In a post on X, U.S. Central Command said some of its Navy guided-missile destroyers were inside the Gulf supporting the operation, and that two U.S.-flagged merchant vessels had crossed the strait "and are safely headed on their journey".

It did not identify either the warships or the merchant vessels or say when any of those crossings had taken place.

Iran's Revolutionary Guards said no commercial vessels had crossed the strait in the past few hours, and that U.S. claims to the contrary were false.

Earlier, Iran said it had fired on a U.S. warship approaching the strait, forcing it to turn around. An initial Iranian report had said a U.S. warship was struck, but Washington denied this and Iranian officials later described the fire as warning shots.

South Korea's Foreign Ministry said there was a fire and an explosion onboard the Namu, a merchant ship operated by South Korean shipper HMM. Yonhap news agency reported that the government was checking intelligence indicating the vessel may have been attacked.

The UAE, meanwhile, reported a fire at an oil installation in its port of Fujairah following an Iranian drone attack. Fujairah lies beyond the strait, making it one of the few export routes for Middle East oil that does not require passing through it.

SHIPPING INDUSTRY AWAITS CLARITY ON SAFETY

Oil prices jumped more than 5% in volatile trade as news of the increased Iranian attacks emerged. [O/R]

In his social media post announcing the new mission, Trump gave few details of what action the U.S. Navy would take to get ships through the strait.

"We have told these Countries that we will guide their Ships safely out of these restricted Waterways, so that they can freely and ably get on with their business," Trump wrote.

In response, Iran's unified command told commercial ships and oil tankers:

"We have repeatedly said the security of the Strait of Hormuz is in our hands and that the safe passage of vessels needs to be coordinated with the armed forces ... We warn that any foreign armed forces, especially the aggressive U.S. Army, will be attacked if they intend to approach and enter the Strait of Hormuz."

The United States and Israel suspended their bombing campaign against Iran four weeks ago, and U.S. and Iranian officials held one round of face-to-face talks. But attempts to set up further meetings have failed.

Iranian state media said on Sunday that Washington had conveyed its response to a 14-point Iranian proposal via Pakistan, and that Tehran was now reviewing it.

Neither side gave details of any U.S. response.

The Iranian proposal would postpone discussion of Iran's nuclear programme until after an agreement to end the war and resolve the standoff over shipping. Trump said over the weekend he was still studying it but would probably reject it.

(Reporting by Reuters bureaux; Writing by Gareth Jones and Peter GraffEditing by Clarence Fernandez, Kevin Liffey and Joe Bavier)

Copyright Reuters or USA Today Network via Reuters Connect.

This story was originally published May 4, 2026 at 1:42 PM.

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