Armed troops with Iran’s Revolutionary Guard stormed a South Korean oil tanker and forced it to change course to Iran, the vessel’s owner said Tuesday, as a delegation from the Asian country prepares to travel to the Islamic Republic to negotiate the crew’s release.

The seizure of the tanker on Monday adds to the heightened tensions in the region over Iran’s nuclear program and the first anniversary of the killing of Gen. Qassem Soleimani, Iran’s top military leader, in a US drone strike a year ago.

Iran said Monday it would step up the process of enriching uranium by 20 percent, a significant development in its effort to obtain a nuclear weapon.

An official at DM Shipping Co. Ltd. told the Associated Press that the Hankuk Chemi with a crew of 20 was traveling from Jubail, Saudi Arabia, to Fujairah in the United Arab Emirates when Iranian forces said they were going to board the ship.

At first, the Iranians said they wanted to conduct an unspecified check, but as the vessel’s captain spoke to company security officials in South Korea, armed forces stormed the tanker.

As an Iranian helicopter hovered overhead, the troops demanded the captain head for Iranian waters without explanation.

The captain has so far been unreachable, and security cameras on the ship’s deck have been turned off, the official said.

Iran said it stopped the Hankuk Chemi because it was polluting the Persian Gulf and the Strait of Hormuz.

The seizure comes as Seoul and Tehran are negotiating over the release of billions of dollars in Iranian assets frozen in South Korean banks, part of the US’ maximum pressure campaign targeting Iran.

An Iranian government spokesman alluded to that standoff when asked about the vessel.

“If anybody is to be called a hostage taker, it is the South Korean government that has taken our more than $7 billion hostage under a futile pretext,” spokesman Ali Rabiei said.

Iran’s central bank recently said it wanted to use the funds locked in a South Korean bank to buy coronavirus vaccines through an international program.

South Korea’s foreign ministry said it would send a delegation for talks about releasing the crew and the ship.

Foreign Minister Kang Kyung-wha wouldn’t say whether she thought Iran was using the seizure as leverage in the negotiations.

“We need to verify the facts first and ensure the safety of our crew,” she said, the BBC reported. “We are making diplomatic efforts for an early release.”

The crew is made up of sailors from Indonesia, Myanmar, South Korea and Vietnam.

The Defense Ministry said it was dispatching an anti-piracy unit — a destroyer and 300 troops — to the Strait of Hormuz.

South Korea’s presidential office said it views the seizure “very gravely.”

The US State Department accused Iran of threatening “navigation rights and freedoms” in the Persian Gulf “to extort the international community into relieving the pressure of sanctions.”

The department called for Tehran to immediately release the tanker.

President Trump in 2018 withdrew the US from the Iran nuclear accord brokered by President Barack Obama and reinstituted crippling financial sanctions.

Some Democrats and the nations still participating in the accord have been pushing for President-elect Joe Biden to rejoin the agreement.

The US has recently ramped up its military presence in the region, sending B-52 bombers flying over the Persian Gulf and a nuclear-powered submarine into the Strait of Hormuz.

On Sunday, Trump overruled the Pentagon and ordered the aircraft carrier USS Nimitz to remain in the Persian Gulf.