Iran denies it is using a South Korean ship and its crew as hostages, a day after seizing the tanker in the Gulf while pressing a demand for Seoul to release billions in funds frozen under US sanctions.
The seizure of the MT Hankuk Chemi and its 20-member crew near the strategic 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 United States.
Iran wants Biden to lift sanctions imposed by outgoing President Donald Trump.
Tehran's critics have long accused it of capturing ships and foreigners as a method of gaining leverage in negotiations.
"We've become used to such allegations," Iranian government spokesman Ali Rabiei told a news conference on Tuesday.
"But if there is any hostage-taking, it is Korea's government that is holding $US7 billion ($A9.0 billion), which belongs to us, hostage on baseless grounds."
South Korea summoned the Iranian ambassador, called for the ship to be released and said it was dispatching a delegation to Iran to discuss it.
Iran says the ship was held over environmental violations.
Iran's ability to challenge shipping in the Gulf is one of its main points of leverage in what is expected to be a difficult negotiation when the Biden administration takes office on January 20.
In 2019, Iran held a British tanker for two months.
South Korea, like other countries, is required to limit Iran's access to its financial system under US sanctionsimposed by Trump after he abandoned a nuclear agreement reached with Iran under his predecessor Barack Obama.
Iran says the sanctions are illegal and have hurt its economy, including its ability to respond to the worst outbreak of COVID-19 in the Middle East.
Biden aims to revive the nuclear agreement but any thaw is likely to pose a diplomatic challenge.
Since Trump abandoned the deal, Iran has taken steps that violate it; Biden says Iran must be fully compliant before the deal can be restored, while Iran says Washington must first lift sanctions.
On Monday, Tehran announced it had stepped up uranium enrichment at an underground facility, its latest move in violation of the nuclear deal's terms.
South Korean Foreign Minister Kang Kyung-wha said on Tuesday she was making diplomatic efforts to secure the release of the tanker and had contacted her counterpart in Tehran.
Iran's ambassador in Seoul, Saeed Badamchi Shabestari, asked about the status of the crew before his foreign ministry meeting, said all were safe.