SEOUL: Iran on Friday released a South Korean-flagged tanker it seized amid a dispute over billions in frozen oil funds, and the vessel's captain, the foreign ministry in Seoul said.

The Islamic Revolutionary Guard Corps seized the Hankuk Chemi and its multinational crew of 20 sailors in January, but South Korea's foreign ministry said in a statement that its detention had been lifted and "the ship departed safely today".

Ship-tracking websites showed the 147-metre-long (482-foot) vessel under way at 9 knots and heading for the Strait of Hormuz.

At the time of the seizure Tehran said the vessel was carrying 7,200 tons of "oil chemical products" and accused it of "repeated infringement of maritime environmental laws".

The move came after Tehran had urged Seoul to release billions of dollars of Iranian assets frozen in South Korea under US sanctions over its nuclear programme.

Tehran said in February that all the crew except the captain would be allowed to leave the country as a humanitarian gesture, but most of them remained on board to maintain the ship.

In the same month Seoul said the two had agreed on a way forward to release billions of dollars frozen from Iran's oil sales -- but that it was awaiting US approval.

Iran was a key oil supplier to resource-poor South Korea until Washington's rules blocked the purchases, and Tehran says it has a total of $7 billion of funds blocked in Seoul.