SEOUL (AP) — Armed Iranian Revolutionary Guard troops stormed a South Korean tanker and forced the ship to change course and travel to Iran, the vessel’s owner said Tuesday, the latest maritime seizure by Tehran amid heightened tensions with the West over its nuclear program.

The military raid on Monday on the MT Hankuk Chemi was at odds with Iranian explanations that they stopped the vessel for polluting the waters of the Persian Gulf and the Strait of Hormuz. Instead, it appeared the Islamic Republic sought to increase its leverage over Seoul ahead of negotiations over billions of dollars in Iranian assets frozen in South Korean banks amid a U.S. pressure campaign targeting Iran.

Iran on Monday also began enriching uranium up to 20%, a small technical step away from weapons-grade levels of 90%, at its underground Fordo facility. That move appeared aimed at pressuring the U.S. in the final days of President Donald Trump’s administration, which unilaterally withdrew from Tehran’s nuclear deal with world powers, and ahead of the inauguration of President-elect Joe Biden, who has said he’d be willing to re-enter the accord.

An official at DM Shipping Co. Ltd. of Busan, South Korea, who spoke to The Associated Press on condition of anonymity as he wasn’t authorized to talk to journalists, offered details of the Hankuk Chemi’s seizure. The vessel had been traveling from Jubail, Saudi Arabia, to Fujairah in the United Arab Emirates when Iranian forces reached the ship and said they would board it.

Initially, Iranian forces said they wanted to run an unspecified check on the ship, the official said. As the vessel’s captain spoke to company security officials back in South Korea, armed Iranian troops stormed the tanker as an Iranian helicopter flew overhead, the official said. The troops demanded the captain sail the tanker into Iranian waters over an unspecified investigation and refused to explain themselves, the official added.

The company has since been unable to reach the captain, the official said. Security cameras installed on the ship that initially relayed footage on the scene on the deck to the company are now turned off, the official said.

After the company lost contact with the captain, the company received an anti-piracy security alert notice, suggesting the captain activated an onboard warning system, the official said. It remains unclear if the ship tried to call for outside assistance.

The U.S. Navy’s Mideast-based 5th Fleet routinely patrols the area along with an American-led coalition monitoring the Strait of Hormuz, the narrow mouth of the Persian Gulf through which 20% of the world’s oil passes. A separate European-led effort also operates there as well.

The official denied the vessel had been polluting the waters.

In past months Iran has sought to escalate pressure on South Korea to unlock some $7 billion in frozen assets from oil sales earned before the Trump administration tightened sanctions on the country’s oil exports.

The head of Iran’s central bank recently announced that the country was seeking to use funds tied up in a South Korean bank to purchase coronavirus vaccines through COVAX, an international program designed to distribute COVID-19 vaccines to participating countries.

South Korea’s Foreign Ministry demanded the ship’s release, saying in a statement that its crew was safe. The crew included sailors from Indonesia, Myanmar, South Korea and Vietnam, according to the Iranian Revolutionary Guard. South Korea’s Defense Ministry said it was sending its anti-piracy unit to near the Strait of Hormuz — a 4,400-ton-class destroyer with about 300 troops.

The U.S. State Department called for the tanker’s immediate release, accusing Iran of threatening “navigational rights and freedoms” in the Persian Gulf in order to “extort the international community into relieving the pressure of sanctions.”

Last year, Iran similarly seized a British-flagged oil tanker and held it for months after one of its tankers was held off Gibraltar.

The latest incidents coincide with the anniversary of the U.S. drone strike that killed Guard Gen. Qassem Soleimani in Baghdad last January. Iran responded by launching ballistic missiles at U.S. bases in Iraq, injuring dozens of U.S. troops. Tehran also accidentally shot down a Ukrainian passenger jet that same night, killing all 176 people on board.

As the anniversary approached and fears grew of possible Iranian retaliation, the U.S. dispatched B-52 bombers over the region and ordered a nuclear-powered submarine into the Persian Gulf.

Acting U.S. Defense Secretary Christopher Miller said late Sunday that he changed his mind about sending the aircraft carrier USS Nimitz home from the Middle East and instead will keep the vessel on duty. He cited Iranian threats against Trump and other U.S. government officials as the reason for the redeployment, without elaborating.

Last week, sailors discovered a limpet mine stuck on a tanker in the Persian Gulf off Iraq, near the Iranian border, as it prepared to transfer fuel to another tanker owned by a company traded on the New York Stock Exchange. No one has claimed responsibility for placing the mine, though it comes after similar attacks in 2019 near the Strait of Hormuz that the U.S. Navy blamed on Iran. Tehran denied involvement.

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Gambrell reported from Dubai, United Arab Emirates. Associated Press writers Isabel DeBre in Dubai; Tia Goldenberg in Tel Aviv, Israel; and Robert Burns and Matthew Lee in Washington contributed to this report.