Iran's navy seized "an American oil tanker" in the Gulf of Oman on Thursday, Iran's official Islamic Republic News Agency reported, amid rising tension in the waters of the Middle East.
Irna reported that the vessel "was stolen by the US and changed its name".
Private intelligence firm Ambrey identified the ship as the Marshall Islands-flagged St Nikolas, which was earlier named the Suez Rajan, and was at the centre of a dispute between Iran and the US over alleged oil smuggling last year.
Iran seized the ship on Thursday in retaliation for a "violation committed by the Suez Rajan ship … and the theft of Iranian oil by the United States", Irna reported.
The US responded to news of the event on Thursday, demanding the release of the ship.
"The Iranian government must immediately release the ship and its crew," said Vedant Patel, deputy spokesman at the US State Department.
"This unlawful seizure of a commercial vessel is just the latest behavior by Iran or enabled by Iran, aimed at disrupting international commerce."
The Pentagon claimed the ship seizure was a breach of international law.
"It's another example of Iranian malign activity, threatening security and stability in the region, and we call on Iran to release the the tanker and the crew immediately," said Pentagon spokesman Maj Gen Pat Ryder.
Earlier on Thursday, the UK Marine Trade Operations agency reported "unauthorised persons" in military uniform boarded a vessel off the coast of Oman.
The incident took place about 90km east of Sohar, the agency said on X, formerly Twitter.
"CSO [the shipping company's security officer] reports hearing unknown voices over the phone along with the master's voice," it added. "Unable to make further contact with the vessel at this time."
Ambrey said men covered surveillance cameras as they boarded the vessel in waters between Iran and Oman.
A representative, speaking anonymously to Reuters, said the vessel had a crew of 19 – one from Greece and 18 from the Philippines – and was carrying 131,000 tonnes of oil from Basra, Iraq, to Aliaga, Turkey.
They confirmed CSO reports that communication with the vessel had been lost.
The ship had also "altered course towards Iranian waters," the UKMTO said in a later update.
US-Iran dispute over Suez Rajan
The St Nikolas was previously named the Suez Rajan when it was associated with Greek shipping company Empire Navigation.
Before it was bought by Empire Navigation, the Suez Rajan was owned by Los Angeles-based private equity firm Oaktree Capital Management.
In February 2022, the group United Against Nuclear Iran said it suspected the Suez Rajan had carried oil from Iran’s Kharg Island, its main distribution terminal in the Gulf.
The US accused Iran's Islamic Revolutionary Guard Corps of attempting to bypass sanctions by selling Iranian crude oil to China via the Suez Rajan.
In April 2023, the US seized the ship, which was carrying more than 980,000 barrels of Iranian crude oil.
Days later, Iran seized two tankers in the Gulf - the Advantage Sweet and the Niovi - as tensions in the Gulf soared.
The Suez Rajan was unable to unload the Iranian crude for nearly two and half months while in Singapore, over fears of secondary sanctions on vessels used to unload it.
After months, it travelled to the US, where the seized oil was transferred to another vessel off the coast of Texas, reportedly into the control of American authorities.
The ship was renamed the St Nikolas after eventually unloading its cargo.
A US Republican senator said the seizure was a "victory lap" for Iran and showed the "weakness" of US President Joe Biden.
“Iran sees Biden’s weakness as an invitation to violate international law. Now, it’s taking a victory lap over seizing an oil tanker," said Senator Joni Ernst, member of the Senate Armed Service Committee.
According to Ms Ernst, the Biden administration delayed the offloading of the oil for "months" because it feared retaliation from Iran's Islamic Revolutionary Guard Corps.
In September, Empire Navigation pleaded guilty to smuggling sanctioned Iranian crude oil and agreed to pay a $2.4 million fine over a case involving the tanker.
The incident comes amid heightened tensions in Middle Eastern waters following repeated attacks on ships in the Red Sea by Iran-backed Houthi rebels in Yemen, who claim to be targeting vessels bound for Israel in retaliation for its war in Gaza.
Tehran has been accused of directing the attacks in a bid to undermine its arch enemy Israel.
On Wednesday, UK defence secretary Grant Shapps said he had "no doubt whatsoever" that "the Iranians are heavily behind what the Houthis are doing".
The Houthis are currently holding the crew of the Galaxy Leader, a Japanese-operated cargo ship with links to an Israeli company, which the group seized on November 19.
Iran has increasingly resorted to vessel seizures for political leverage since the collapse of the 2015 nuclear deal.
In August, ships transiting the key Strait of Hormuz were warned against straying into Iranian waters due to the "high risk" of seizure.
Iranian attacks have prompted the US to strengthen its naval presence in the region, deploying its first drone boats in the Strait of Hormuz in April.
The US has also established a multinational maritime task force to protect shipping in the Red Sea, known as Operation Prosperity Guardian, in addition to several other maritime task forces it has in the region.
The US Fifth Fleet is also based in Bahrain.
With additional reporting from Reuters and AP