Iran's foreign minister has called on the United States to unlock $10 billion of Tehran's frozen assets to show goodwill, and clear the way for a return to the nuclear deal 2015.

If the Americans have "true intentions, let them release some of our assets, for example, $10 billion frozen in foreign banks", Hossein Amirabdollahian said in an interview with the state television IRIB late Saturday.

"But the Americans are not prepared to unlock them for us to be assured that they've taken into account the interests of the Iranian people at least this one time over the past decades," he said.

Amirabdollahian also warned that Tehran would sue South Korea if it continued to refuse to honor a debt of almost $8 billion for purchases of Iranian oil.

Iran's funds are frozen in South Korean banks.

"US pressure (on Seoul) is a fact but we cannot continue... to turn a blind eye to this question," he said, adding the government would allow the central bank to take legal action against two South Korean banks holding the funds.

Amirabdollahian said he had addressed the issue on Thursday during a telephone conversation with his South Korean counterpart Chung Eui-yong and asked him to allow Iran access to its assets "as soon as possible".