South Korea is dispatching a delegation to Iran, on Tuesday, to seek the release of a tanker seized in the Gulf waters by Iranian forces.

The delegation, with a senior diplomat, is set to go ahead with a planned visit to Tehran on Sunday amid tensions over $7 billion in Iranian funds frozen in Korean banks due to U.S. sanctions.

News of the visits came as Seoul’s Foreign Ministry called in the Iranian ambassador to South Korea for a meeting and urged the early release of the South Korean-flagged tanker and its crew of 20.

It was carrying a cargo of more than 7,000 tonnes of ethanol when it was seized on Monday over what Iranian media said were pollution violations.

The incident comes as Iran shows increasing signs of willingness to assert its claims in the region as U.S. President-elect Joe Biden prepares to take office later this month, succeeding Donald Trump.

On Monday, Tehran also said it had resumed 20 percent uranium enrichment at its underground Fordow nuclear facility.

The Trump administration re-imposed sanctions on Iran in 2018 after Washington withdrew from Tehran’s 2015 nuclear deal with six major powers.

Asked about the status of the ship’s crew before his meeting at the Seoul foreign ministry, Iranian ambassador, Saeed Badamchi Shabestari, told reporters “all of them are safe”.

Iranian state TV previously cited a Tehran government official as saying South Korea’s Vice Foreign Minister, Choi Jong-Kun, had been scheduled to visit before the seizure of the tanker Hankuk Chemi to discuss Iran’s demand that the frozen funds be released.

Vice-minister Choi will discuss “various pending issues” between the two countries on top of the seizure, foreign ministry spokesman, Choi Young-sam, told a briefing in Seoul on Tuesday.

“In the earliest possible time, a working-level delegation led by the regional director will be dispatched to Iran to try to resolve the issue on the ground through bilateral negotiations,’’ spokesman Choi said.

The ship’s Busan-based operator, Taikun Shipping Co. Ltd., told Reuters there was nothing to indicate before the seizure of the vessel that Iranian authorities were probing possible violations of environmental rules.

“If it really was marine pollution, as they (Iranian media) say, the coast guard was supposed to approach the ship first,’’ Taikun’s Management Director, Lee Chun-hee, said by telephone.

“But instead, armed soldiers approached the crew and said they needed to be investigated.’’

South Korea’s Foreign Minister, Kang Kyung-wha, said on Tuesday, she was making diplomatic efforts to secure the release of the tanker and that she had made contact with her counterpart in Tehran, the Yonhap news agency reported.

Tilak Doshi, a visiting senior fellow at Middle East Institute, National University Singapore said the incident provided an example of Iran’s willingness to flex its muscle as Joe Biden prepares to take office and that the frozen assets would likely be raised in talks with Seoul.

“No doubt, in negotiations for the release of the tanker and its crew, the release of the funds by South Korea will be the key demand on the Iranian side,’’ he said.

Last Sunday the Tehran Times newspaper reported Iran was hoping to negotiate an agreement to use the frozen funds to “barter” for vaccine doses in the fight against the global coronavirus pandemic and other commodities.

According to Yonhap, a foreign ministry official said the Iranian government had tried to secure vaccines through the global COVAX initiative, backed by the World Health Organisation.

Tehran had been in talks with the ministry and the U.S. Treasury to pay for the doses with South Korean won, Yonhap said, citing a source in the ministry.