A South Korean delegation is heading to Iran on Wednesday to seek the release of a chemicals tanker and its 20-member crew seized in Gulf waters by Iranian forces, Yonhap news agency reported. Armed Islamic Revolutionary Guard troops stormed the South Korean tanker and forced the ship to change course and travel to Iran, the vessel's owner said on Tuesday, the latest maritime seizure by Tehran amid heightened tension with the West over its nuclear program. Iran denied on Tuesday that it was using the ship and its crew as hostages while pressing a demand for Seoul to release $7 billion in funds frozen under US sanctions. Instead Iranian authorities claimed that they stopped the vessel for polluting the waters of the Arabian Gulf and the Strait of Hormuz. “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. The seizure of the <em>MT Hankuk Chemi</em>, near the Strait of Hormuz, has been seen as an attempt by Tehran to assert its demands, just two weeks before president-elect Joe Biden takes office in the US. Iran wants Biden to lift sanctions that were imposed by outgoing President Donald Trump. Iran on Monday also began enriching uranium up to 20 per cent, a small technical step away from weapons-grade levels of 90 per cent, at its underground Fordo centre. That move appeared aimed at pressuring the US in the final days of President Donald Trump's administration, which unilaterally withdrew from Tehran's nuclear deal with world powers. Seoul's foreign ministry called in the Iranian ambassador on Tuesday to call for the early release of the tanker and its crew of 20. In a report submitted to parliament, the ministry said South Korea is reviewing whether the ship breached international law by polluting the waters, as claimed by Tehran, but also whether Iran breached regulations during the boarding and seizure process. South Korea’s vice foreign minister Choi Jong Kun is scheduled to visit Tehran on Sunday, a visit that was previously scheduled to discuss a range of bilateral issues, the ministry said on Tuesday. In response to the seizure, Japan’s Ministry of Land, Infrastructure, Transport and Tourism warned Japanese shippers to use extra caution in operations in the area, an official at the ministry’s maritime bureau said.