South Korea will send a delegation to negotiate the release of a ship and its 20-member crew after the vessel was seized by Iranian forces, officials said on Tuesday, the latest development in a provocation by the government in Tehran, which has been economically isolated by U.S. sanctions.

Iranian officials said the ship was detained in the Persian Gulf by Iran's Revolutionary Guard because it had violated environmental protocols and was polluting the sea, according to Iranian state news agencies. The ship was carrying 7,200 tons of chemicals, mostly methanol, according to the South Korean company that owns it, which has denied that it was polluting the waters.

The tensions come as Tehran has sought to pressure the government in Seoul to release about $7 billion in revenue from oil sales that remain frozen in South Korean banks since the Trump administration tightened sanctions.

But they also follow escalation of tensions between the U.S. and Iran as President Donald Trump's term draws to a close. Iran said on Monday that it was starting to increase nuclear enrichment levels at a key facility to 20%, a step closer to developing the capacity to produce a nuclear weapon. The Pentagon said on Sunday that it had directed the aircraft carrier Nimitz to remain in the Middle East, days after it had ordered the ship to return home, because of Iranian threats against Trump and other U.S. officials.

Iran rejected allegations on Tuesday that it had seized the South Korean ship as leverage but reiterated its complaint over the locked-up funds. "If anyone is a hostage taker it's South Korea's government, which has taken hostage more than $7 billion of our revenue for no reason," said Ali Rabiei, a government spokesman.

But news outlets linked to the Revolutionary Guard ran front-page headlines linking the seizure of the ship to negotiations with South Korea on releasing the frozen funds. "We captured the thieves," said a headline on the newspaper Vatan Emrooz. "A clean response to revenue thieves," said the Tasnim News Agency.

The ship, the tanker Hankuk Chemi, was sailing from Jubail, Saudi Arabia, to Fujairah in the United Arab Emirates when armed Revolutionary Guard members approached it, according to Ri Il-Su, an official at DM Shipping Co., the company that owns it. Once aboard, they forced the ship to change course and sail to Iran.

"The Iranian troops said they were coming onboard for an investigation but didn't answer questions about what the investigation was about," said Ri, who added that the captain of the ship called the company during the seizure.

Communication between the ship and the company was cut off, and the company received an anti-piracy security alert notice from the ship. Ri called accusations that the vessel was polluting waters "absurd."

The vessel is being held in the port of Bandar Abbas and the case is expected to be referred to the judiciary, Iranian media reported, citing navy authorities. There were five people from South Korea onboard, along with 11 from Myanmar, two from Indonesia and two from Vietnam, according to Choi Young-sam, a spokesman for the South Korean Foreign Ministry. "Our goal is an early release of the ship and its crew," Choi said.

The issue will also be addressed during a planned visit to Tehran by a South Korean vice foreign minister, Choi Jong-Kun, next week.

The future of relations between Iran and the U.S. appears to have reached an inflection point as President-elect Joe Biden prepares to take office this month. A security adviser to Biden said on Sunday that if Iran re-entered compliance with the 2015 nuclear deal that Trump left in 2018, the new administration would pursue "follow-on negotiation" over Iranian missile capabilities.

But Tuesday, Iran accused the U.S. of triggering new tensions in the region, according to Press TV.