The crew of a South Korean tanker seized by Iran are still on board the ship, Seoul said Thursday, despite Tehran announcing earlier this week it would release them in a humanitarian move.

The Islamic Revolutionary Guard Corps seized the Hankuk Chemi and arrested its multinational crew of 20 sailors near the strategic Strait of Hormuz on January 4, saying it had polluted the waters.

The development came as Tehran urged Seoul to release billions of dollars of Iranian assets frozen in South Korea due to US sanctions.

On Tuesday, Iran's foreign ministry said the crew of the seized tanker had been granted permission to leave the country "in a humanitarian move", although it would retain the vessel and its captain.

South Korean reports say the partial release complicates the situation as the ship needs personnel present to maintain it.

All the crew members remained on board, Seoul's foreign ministry said Thursday, adding South Korean officials had been allowed to meet them Wednesday for the first time since the seizure.

"The foreign ministry is discussing with the tanker company on the disembarkation and return of the crew and will put all efforts to secure the release of the captain and the ship as early as possible," it said in a statement.

The South Korean-flagged tanker is escorted by Iran's Revolutionary Guards after being seized in the Gulf, in this picture issued by the Iranian news agency Tasnim on January 4, 2021
The South Korean-flagged tanker is escorted by Iran's Revolutionary Guards after being seized in the Gulf, in this picture issued by the Iranian news agency Tasnim on January 4, 2021 TASNIM NEWS / -

A representative at the tanker company DM Shipping said nothing was certain despite Tehran's announcement the crew would be released.

"Discussions for their return are still ongoing," he told AFP. "It hasn't been decided whether they will return 100 percent."

The arrested crew were from South Korea, Indonesia, Vietnam, and Myanmar.

Former US president Donald Trump in 2018 withdrew Washington from a landmark nuclear agreement with world powers and then reimposed and reinforced crippling sanctions on Iran.

Iran was a key oil supplier to resource-poor South Korea until Washington's rules blocked the purchases.

According to government spokesman Ali Rabiei, Iran has $7 billion of funds blocked in Seoul.

Tehran has repeatedly denied any link between the ship's seizure and the funds issue.

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