Iran on Thursday confirmed its navy had captured a tanker with Iraqi crude destined for Türkiye, in retaliation for the confiscation last year of the same vessel and its oil by the U.S., Iranian state media reported, further escalating the tensions gripping the Mideast's waterways.

The seizure of the Marshall Islands-flagged St. Nikolas in the Gulf of Oman coincides with weeks of attacks by Yemen's Iran-backed Houthi rebels targeting Red Sea shipping routes to show support for Palestinian resistance group Hamas in its fight against Israel.

The St. Nikolas, chartered by the Turkish refinery giant Tüpraş, loaded 145,000 metric tons of oil in the Iraqi port of Basra and was heading to Aliağa in western Türkiye. Tüpraş said it had lost contact with the vessel as of Thursday morning.

The tanker was seized by the United States last year in a sanctions enforcement operation when it sailed under a different name, Suez Rajan. The dispute ultimately saw the U.S. Justice Department take almost 1 million barrels of Iranian crude oil on it.

Following the move, Iran warned the U.S. that it would "not go unanswered."

Iran's state-run television acknowledged the seizure late Thursday afternoon, hours after armed men boarded it, linking it to the earlier oil seizure. It said Iran's Navy, rather than its paramilitary Revolutionary Guard, conducted the seizure. Past tense incidents at sea have largely involved the Guard.

"After the theft of Iranian oil by the United States last year, the St. Nikolas tanker was seized by Iran's Navy this morning with a judicial order... it is en route to Iranian ports," the semi-official Fars news agency reported, citing a statement by the navy.

The "seizure of the oil tanker does not constitute hijacking; rather, it is a lawful undertaking sanctioned by a court order and corresponds to the theft of Iran's very own oil," Iran's mission to the United Nations told The Associated Press (AP) in a statement. "Adhering to the established legal procedures is the most prudent approach for the resolution of this matter."

In a statement to the AP, its operator, Athens-based Empire Navigation, acknowledged losing contact with the vessel, which has a crew of 18 Filipinos and one Greek national.

"Empire have no such knowledge of a court order or the Iranian Navy having seized their vessel, and have still not been contacted by anyone," the company said.

Türkiye's Tüpraş confirmed that St. Nikolas was loaded with Iraqi crude cargo from Basra that was sold by Iraq's state-owned marketer SOMO.

"Communication with the oil tanker, St. Nikolas, under Marshall Islands flag and owned by the Greek shipowner Empire Navigation has been cut off around 06:30 on Jan. 11 in the waters of Oman," it said in a statement.

"The incident has no impact on our refinery operations," Tüpraş, which operates the 241,500 barrel per day (bpd) capacity Izmir refinery in Aliağa, added.

In Washington, the Pentagon said Iranian forces unlawfully boarded the St. Nikolas and forced it to change course toward Iranian territorial waters.

The White House and the U.S. State Department condemned the seizure.

"No justification whatsoever to seize it, none whatsoever. They need to let it go," White House national security spokesperson John Kirby said.

The State Department also called for the immediate release of the ship and its crew.

"This unlawful seizure of a commercial vessel is just the latest behavior by Iran or enabled by Iran aimed at disrupting international commerce," State Department spokesperson Vedant Patel told reporters.

"We believe this kind of action will simply add uncertainty for commercial shipping," said Patel. "Provocative actions like this are a menace to the global economy and must cease."

Thursday's seizure began early in the morning in the waters between Oman and Iran in an area transited by ships coming in and out of the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a fifth of all traded oil passes, the British military's United Kingdom Maritime Trade Operations, which provides warnings to sailors in the Middle East, said.

The U.K. military-run group described receiving a report from the ship's security manager of hearing "unknown voices over the phone" alongside with the ship's captain. It said further efforts to contact the ship had failed and that the men who boarded the vessel wore "black military-style uniforms with black masks."

The private security firm Ambrey said that "four to five armed persons" boarded the ship, which it identified as the oil tanker St. Nikolas. It said the men covered the surveillance cameras as they boarded.

Ambrey said the recently renamed tanker was previously prosecuted and fined for carrying sanctioned Iranian oil. The Suez Rajan was carrying more than 980,000 barrels of Iranian crude oil last year when it was seized and the oil confiscated by the U.S.

The U.S. said at the time that Iran's Islamic Revolutionary Guard Corps (IRGC) had been trying to send "contraband" Iranian oil to China, in violation of U.S. sanctions.

Iran has been under crippling U.S. sanctions reimposed following Washington's 2018 withdrawal from a landmark nuclear deal. The sanctions target Iranian oil and petrochemical sales in a bid to reduce Iran's energy exports.

Iran has responded with tit-for-tat measures in the past after the oil confiscations.

Shortly after the seizure last year, Iran captured two tankers – the Marshall Islands-flagged Advantage Sweet as it sailed toward the U.S. in the Gulf of Oman, and then the Greek-owned Niovi, as it traveled from Dubai to Fujairah.

The Gulf of Oman, a key route for the oil industry that separates Oman and Iran, has witnessed a series of seizures and attacks over the years, often involving Iran.

Shipping in the resource-rich region is on heightened alert following weeks of drone and missile attacks by Yemen's Houthi rebels on shipping in the Red Sea, including their largest barrage ever of drones and missiles launched late Tuesday.

That has raised the risk of possible retaliatory strikes by U.S.-led forces now patrolling the vital waterway, especially after a United Nations Security Council vote on Wednesday condemning the Houthis, and as American and British officials warned of potential consequences over the attacks.

IRNA news agency, quoting the Iranian Navy's public relations office, said St. Nikolas was "being transferred to the ports of the Islamic Republic for delivery to the judicial authorities."

Satellite-tracking data analyzed by AP last showed the tanker had turned and headed toward the port of Bandar-e Jask in Iran.