Two oil tankers have reportedly caught fire after being attacked in the Gulf of Oman.
Taiwan’s state oil refiner CPC Corp said one of them, the Front Altair, was “suspected of being hit by a torpedo”.
Maritime intelligence firm Dryad Global said it was “on fire and adrift”.
The fire was caused by an explosion, International Tanker Management, which operates the vessel said.
Oil tanker association Intertanko confirmed that the ships had been attacked.
It added that oil supplies to the West could be at risk if waters around the Strait of Hormuz, which the Gulf of Oman leads into, were to become unsafe. A fifth of global oil consumption passes through the Strait of Hormuz.
Iran’s Islamic Republic News Agency (IRNA) reported that the Front Altair had sunk.
But Norwegian shipping firm Frontline said it was still afloat.
“The vessel hasn’t sunk,” company spokesman Pat Adamson said. “There are pictures from a salvage vessel that is circling the ship.”
The other tanker – Kokuka Courageous – was attacked twice in three hours, the president of Japanese owner Kokuka Sangyo told reporters, and breached above the waterline, igniting a fire in an engine room.
The UK is urgently seeking to establish the facts, the prime minister’s spokesman said.
United Kingdom Maritime Trade Operations – part of the Royal Navy – said it was investigating.
Both vessels were carrying “Japan-related” cargo, Japan’s Trade Ministry said.
Iran’s foreign minister, Mohammad Javad Zarif, called for regional talks after describing the incidents as “suspicious”.
Mr Zarif tweeted that “reported attacks on Japan-related” tankers had taken place while Japanese Prime Minister Shinzo Abe was meeting Iran’s Supreme Leader Ayatollah Khamenei “for extensive and friendly talks”.
He added: “Suspicious doesn’t begin to describe what likely transpired this morning.”
All major Gulf stock markets dropped following the news, and oil prices rose by about 4%.
Fire broke out on the Front Altair, which was heading from Qatar to Taiwan, about 25 miles from the Iranian port of Jask, IRNA said.
It was carrying 75,000 tonnes of a liquid hydrocarbon mixture called naphtha when it was attacked at 5am UK time, CPC Corp said. The cargo’s value is estimated at $ 30m (23.7m).
Petrochemical division CEO, Wu I-Fang, said all 23 crew had been rescued.
The Kokuka Courageous was carrying methanol.
It sustained damage to its starboard hull, Bernhard Schulte Shipmanagement said, following a “suspected attack” that breached the hull.
The engine room fire is said to have broken out about 28 miles from Jask.
One crew member was slightly injured.
Iranian search and rescue teams picked up 44 sailors from the two tankers and took them to Jask, IRNA reported.
Another Iranian news agency, Fars, said some of them were given medical help.
The US navy is providing assistance, saying it was “aware of the reported attack” and had received “two separate distress calls”.
This latest incident follows allegations from the US that Iran used mines to attack four oil tankers off the Emirati port of Fujairah last month.
Iran has denied being involved.