Bagheri made the comment during a meeting with Seoul's Vice Foreign Minister Choi Jong-kun in the Austrian city, where talks continue between Tehran and world powers aimed at restoring a 2015 nuclear agreement.

"Whatever the result of the negotiations in Vienna, the South Korean government has a duty to unblock Iran's frozen funds, and the unilateral American sanctions cannot justify non-payment of these debts," Bagheri said, as cited by the official news agency IRNA.

The parties to the 2015 agreement with Iran saw it as the best way to stop the Islamic republic from building a nuclear bomb -- a goal Tehran has always denied.

It offered Iran much-needed relief from economic sanctions and curtailed the country's nuclear activities.

But the unilateral withdrawal by then-US president Donald Trump in 2018 derailed the pact.

Tehran started reneging on its commitments after Trump reimposed sanctions including a unilateral ban on oil sales.

Iran had been one of South Korea's main suppliers of crude.

But last year a row between the two countries intensified, with Tehran threatening legal action unless Seoul released the more than $7 billion in frozen funds for oil shipments.

A foreign ministry official in Seoul earlier told AFP that "it is difficult to confirm" the exact amount of money involved.

IRNA said the meeting was not "directly linked to the negotiations underway" between Iran and the international powers, and took place at the "request" of South Korea.

In a tweet on Thursday, Choi said he "had a good meeting" with Bagheri in Vienna.

"We exchanged views on our bilateral relationship including the frozen fund," he said. "Korea and Iran will work together and preserve our historically important relationship."

In a statement on Tuesday, South Korea's foreign ministry said its delegation aimed to "explore ways to resolve the issue of frozen Iranian assets in Korea on the sidelines" of the nuclear talks.

In addition to consultations with Iran, this would involve "close coordination and communication" with the US, European Union, France, Germany and the United Kingdom, the South Korean statement said.

France, Germany and the UK are participating in the talks about the 2015 deal, along with Russia and China, while the US is indirectly involved.