At least two merchant vessels attempting to transit the Strait of Hormuz have come under fire.
The incident follows Iran's decision to close the critical shipping lane, with Tehran claiming Washington has failed to honour its commitments under a ceasefire deal.
Prior to the closure, multiple ships had begun navigating the strait in the "first major movement" of maritime traffic through the waterway since hostilities began.
The UK Maritime Trade Operations has confirmed it logged a reported incident in waters northeast of Oman, near the strait.
According to the Royal Navy-run organisation, a tanker captain reported that vessels belonging to Iran's Revolutionary Guard approached his ship before opening fire.
Despite the gunfire, both the tanker and its crew are understood to be safe.
The incident was recorded by UK Maritime Trade Operations, which monitors shipping activity in the region.
Ships were reportedly informed that the key waterway was "completely open" following Donald Trump's announcement on Friday evening.
At least two merchant vessels attempting to transit the Strait of Hormuz have come under fire
|REUTERS
However, despite hopes that the relationship between Tehran and Washington was improving, the Iranian military said in a statement that the strait had "returned to its previous state" and that Iran's approval is required to travel through it.
The statement from Iran's Central Military HQ said: "Iran agreed to allow a limited number of ships to pass through the Strait of Hormuz according to agreements.
"But the US did not fulfil their obligations. So, the Strait of Hormuz is now closed again and passage requires IRAN approval."
The passage of water has been effectively closed since the start of America's attack on February 28.
MAPPED: Where is the Strait of Hormuz? | GB NEWSMarineTraffic monitoring showed multiple vessels navigating the passage this morning, including carriers designed for oil, liquefied petroleum gas and chemical cargo.
The movement represents the first significant transit of commercial shipping through the strait since hostilities commenced seven weeks ago.
Since the United States imposed a blockade on Iranian ports and coastal areas, 23 ships have complied with US forces orders to turn back toward Iran, the US military said.
The UK and France have said they will "lead a multinational mission" to protect vessels travelling through the Strait of Hormuz "as soon as conditions allow".
A satellite image shows the ship movement at the Strait of Hormuz during its closure
|REUTERS
Speaking from Paris yesterday, Sir Keir Starmer said: "I can confirm that along with France, the UK will lead a multinational mission to protect freedom of navigation as soon as conditions allow.
"This will be strictly peaceful and defensive, as a mission to reassure commercial shipping and support mine clearance.
"We invite all nations with an interest in the free flow of global trade to join us. Some have already indicated their readiness to contribute.
"We will take this forward with a military planning conference in London next week where we will announce more detail on the composition of the mission, and over a dozen countries have already offered to contribute assets."
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