Iran's navy said it had prevented the entry of enemy warships into the Strait of Hormuz by issuing a "swift and decisive warning", according to state TV.
US President Donald Trump said yesterday that the United States would start an effort this morning to free ships stranded in the strait as a "humanitarian gesture" to aid neutral countries in the US-Israeli war with Iran.
The semi-official Fars news agency said a US warship had been hit by two missiles while sailing near the port of Jask at the southern entrance to the strait and turned back from its attempt to transit the strait.
Iran's armed forces issued several warnings to foreign navies not to cross into the Strait of Hormuz or face a "decisive response".
A senior US official has denied that a US ship was hit by Iranian missiles, Axios reported.
Separately, authorities in the United Arab Emirates sent an emergency phone alert warning of potential missile threats and asking residents to immediately seek a safe place and await further instructions, witnesses said.
It was the first alert to be sent by the authorities in recent weeks.
Authorities send a second message a short time later telling residents that the situation was safe.
Mr Trump gave few details of the plan to aid ships and their crews that have been "locked up" in the vital waterway and are running low on food and other supplies more than two months since the conflict began.
"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," Mr Trump said in a post on his Truth Social site yesterday.
The unified command of Iran's armed forces responded by warning US forces to stay out of the strait.
Its forces would "respond harshly" to any threat, it added, telling commercial ships and oil tankers to refrain from any movement in the absence of coordination with Iran's military.
"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," Ali Abdollahi, the head of the forces' unified command said in the statement.
"We warn that any foreign armed forces, especially the aggressive US army, will be attacked if they intend to approach and enter the Strait of Hormuz."
US Central Command said it would support the effort with 15,000 military personnel, more than 100 land and sea-based aircraft, along with warships and drones.
"Our support for this defensive mission is essential to regional security and the global economy as we also maintain the naval blockade," Admiral Brad Cooper, the CENTCOM commander, said in a statement.
Hundreds of ships and as many as 20,000 seafarers have been unable to transit the strait during the conflict, the International Maritime Organization says.
Soon after Mr Trump's comments, the United Kingdom Maritime Trade Operations agency said a tanker had reported being hit by unknown projectiles in the strait.
The agency said all crew were reported safe in the incident, which occurred 78 nautical miles north of Fujairah, in the United Arab Emirates, but few details were immediately available.
Iran has been blocking nearly all shipping from the Gulf apart from its own for more than two months, sending energy prices soaring.
Some vessels attempting to transit the strait have reported being fired on, and Iran seized several other ships. Last month, the US imposed its own blockade of ships from Iranian ports.
The Trump administration has been seeking help from other countries to form an international coalition to secure shipping in the strait. CENTCOM said the latest effort would combine "diplomatic action with military coordination."
It was not immediately clear which countries the US operation would aid or how the operation would work.
It will not necessarily include US Navy ships escorting commercial ships, Axios reporter Barak Ravid said in a post on X.
Mr Trump threatened that any interference with the US operation would "have to be dealt with forcefully."
Equity markets edged higher this morning while crude oil prices were little moved, having surged back above $100 a barrel last week amid uncertainty over when and how the conflict will be resolved.
Yesterday, Iran said it had received a US response to its latest offer for peace talks a day after Mr Trump said he would probably reject the Iranian proposal because "they have not paid a big enough price."
Mr Trump, responding to shouted questions from reporters, said last evening that talks were going "very well", without elaborating.
Iranian state media said the US had conveyed its response to Iran's 14-point proposal via Pakistan, and that Tehran was now reviewing it. There was no immediate confirmation from Washington or Islamabad of the US response.
"At this stage, we do not have nuclear negotiations," state media quoted Iran's foreign ministry spokesperson Esmaeil Baghaei as saying.
The comment was an apparent reference to Iran's proposal to set aside talks on nuclear issues until after the war has ended and the foes have agreed to lift opposing blockades of Gulf shipping.
The United States and Israel suspended their bombing campaign against Iran four weeks ago, and US and Iranian officials held one round of talks. But attempts to set up further meetings have so far failed.
The proposal to delay talks on nuclear issues until a later phase would appear at odds with Washington's repeated demand that Iran accept stringent restrictions on its nuclear programme before the war can end.
The US wants Iran to give up its stockpile of more than 400kg of highly enriched uranium, which the United States says could be used to make a bomb.
Iran says its nuclear programme is peaceful, although it is willing to discuss some curbs in return for the lifting of sanctions. It had accepted such curbs in a 2015 deal that Mr Trump abandoned.
While repeating he is in no hurry, Mr Trump faces domestic pressure to break Iran's hold on the Strait of Hormuz, which has choked off 20% of the world's oil and gas supplies and driven up US gasoline prices.
His Republican Party faces the risk of a voter backlash over higher prices in midterm congressional elections due in November.
Iranian media said Tehran's 14-point proposal includes withdrawing US forces from nearby areas, lifting the blockade, releasing frozen assets, paying compensation, lifting sanctions, ending the war on all fronts including Lebanon, and creating a new control mechanism for the strait.
