Iran’s Navy said it had seized a tanker loaded with crude oil off the coast of Oman on Thursday, apparently in retaliation for the United States confiscating oil from the same ship last year.

A British government maritime agency said the ship was reportedly boarded by four or five armed people wearing uniforms and black masks. Iranian forces said the boarders arrived by helicopter, and released video that they said showed the men descending from a craft hovering above the ship’s deck.

The U.S. government diverted the same vessel last year and confiscated its cargo, Iranian oil that was being transported in violation of American sanctions. The ship, then called the Suez Rajan, was later renamed the St. Nikolas.

The Iranian Navy claimed on Thursday in a statement that the tanker “stole Iran’s oil under the order of the U.S. and transported it to American shores,” and said it had impounded the ship with an order from the Iranian judiciary.

Iran claimed that the ship was American, but the St. Nikolas is registered in the Marshall Islands, and the company that operates it, Empire Navigation, said it was not American. U.S. court documents state the ship is leased by a Marshall Islands company, and that Empire is also incorporated there, and operates in Greece.

The ship was taken to Bandar Jask, a small Iranian port on the Sea of Oman, where the crew — 18 Filipino nationals and one Greek national, Empire said — will remain on board, according to an official from Iran’s oil ministry, who spoke on condition of anonymity because he was not authorized to speak publicly. Empire said the tanker was carrying about 1 million barrels of Iraqi crude oil and was headed for Turkey.

Iran plans to hold the ship and the crew until Iran is repaid for a similar amount of oil — valued at about $75 million at current prices — that the United States seized from it last year, according to the oil ministry official and a Tehran-based oil dealer who works with the ministry and was also not authorized to speak publicly. The official said Iran sees the U.S. confiscation as theft.

Iran has seized tankers before, and declared last summer that it would retaliate for the American taking of the Suez Rajan. But the seizure of the St. Nikolas by Iran comes at a particularly sensitive time, just as the United States and its allies, including Israel, are confronting Iranian-backed militias in the Red Sea, the Gaza Strip, Lebanon, Syria and in Iraq.

While the United States, Britain and other Western countries are trying to prevent the Israel-Hamas war from spreading on other fronts, they are also weighing whether to carry out military strikes against the Iranian-backed Houthi militia, which has been attacking Red Sea shipping.

Last year, U.S. federal prosecutors filed a criminal case against Empire Navigation and Suez Rajan Limited, incorporated in the Marshall Islands and described as the “bareboat charterer” — the entity that effectively leases the ship, but then hires an operator like Empire to manage it.

The U.S. government charged that in 2022, the Suez Rajan took on about 980,000 barrels of Iranian crude from another tanker, in violation of sanctions, and that shipping and financial records were falsified to conceal the origin of the oil and the fact that the seller was Iran’s Revolutionary Guards Corps.

Cooperating with the prosecution, the ship diverted to American waters, and sat off the coast of Galveston, Texas, for several months while the government looked for a company willing to offload the oil and incur the risk of Iranian retaliation.

Suez Rajan Limited pleaded guilty to conspiracy to violate sanctions law and agreed to pay a fine of almost $2.5 million, and Empire accepted joint liability for the fine. The ship was renamed and resumed sailing.

The Iranian oil ministry official said that the navy had carried out the raid, rather than the Revolutionary Guards, and had emphasized the judicial order — it was unclear from what court — to minimize resulting tensions with the United States. The message, the official said, is that Iran sees its actions as commercial rather than political, and wants to be repaid and to deter further seizures of its oil.

The Pentagon spokesman, Maj. Gen. Patrick Ryder, called on Iran “to release the tanker and the crew immediately.”

“This activity is contrary to international law,” he added. “It’s another example of Iranian malign activity, threatening security and stability in the region.”

On Wednesday, U.S. and British officials said they had intercepted one of the largest barrages yet of drones and missiles fired from an area in Yemen controlled by the Houthis. The Houthis have refused to back down, despite threats of retaliation from Washington and its allies.

The St. Nikolas was seized Thursday morning off the port of Sohar on Oman’s northern coast, near the Strait of Hormuz, according to the British agency, United Kingdom Maritime Trade Operations.