Two US officials told Reuters that Iran had fired on the cargo ship near Oman, while Iran's Persian Gulf Strait Authority, said vessels outside routes it has set will not be guaranteed safe passage. "Consequences arising from passage through unauthorised routes shall be the responsibility of the owner, operator, and vessel commander," the Iranian authority said.
Four sources identified the ship as the Singapore-flagged Ever Lovely. A security source said it was likely targeted by a drone. There was no immediate comment from the US government.
President Donald Trump warned earlier this month that if Iran did not honour an agreement aimed at ending the war and reopening the strait, the US would probably go back to bombing the country again.
The IMO was helping to get hundreds of ships and thousands of seafarers out of the strait where they had been stranded for months since the start of the war in late February.
It decided "to temporarily pause its implementation in order to reconfirm that the necessary safety guarantees continue to be in place for the ships on our evacuation list and all those in the region," IMO Secretary General Arsenio Dominguez said in a statement.
The IMO said the ship involved in the suspected attack was not part of its evacuation program.
The initiative, which was launched on Tuesday, was a voluntary option for ships and their crew to sail out of the Gulf using two routes – one via Iranian waters and the other via Omani waters, with US oversight.
Benchmark oil prices rose 2 per cent following reports of the attack, which analysts said rekindled concerns about how long it could take for Gulf oil flows to resume normal levels.
Before the incident, US Secretary of State Marco Rubio - wrapping up a tour of the Gulf to reassure states about the interim pact - told reporters that if Iran threatens or blocks ships in the strait, "then we're going to have a problem." Iran, though, has signalled it would continue to assert control over the strait.
Earlier, US Energy Secretary Chris Wright said shipments through the strait were approaching levels seen before the US and Israel launched strikes on Iran on February 28, with at least 20 million barrels of oil exiting the waterway in the previous 24 hours. Other shipping data showed crude shipments through the Strait of Hormuz rose this week to their highest level since the war began in February, while South Korea's Oceans Ministry said eight more South Korean vessels have exited the strait.
Saudi Aramco has resumed oil loading at its Ras Tanura terminal in the Gulf after a near four-month halt, shipping data shows.
The Saudi energy major had to divert all its exports to the Red Sea port of Yanbu during Iran's blockade of the Strait of Hormuz.
The war is weighing heavily on Trump ahead of November midterm elections that will determine control of Congress. Just one in four Americans believes the war was worth the cost, a Reuters/Ipsos poll showed. Conflicting accounts have emerged over elements of the framework ceasefire deal, which has prompted criticism of Trump at home and abroad.
Disagreements persist over financial incentives for Iran, nuclear inspections, control of the Strait of Hormuz and Israel's parallel war in Lebanon.
Iran's top negotiator, Mohammad Baqer Qalibaf, said on Thursday that the US assertion that Iran would spend its unfrozen assets to buy US agricultural products was false.
