Iranian Foreign Minister Hossein Amirabdollahian has called on the US to unlock $10 billion of Tehran's frozen assets to clear the way for a return to a nuclear deal with major powers.
If the Americans have "true intentions, let them release some of our assets, for example $10bn frozen in foreign banks", Mr Amirabdollahian told state television.
"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."
Mr Amirabdollahian also warned that Tehran would sue South Korea if it continued to refuse to honour a debt of almost $8bn 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.
Mr Amirabdollahian said Tehran would allow the central bank to take legal action against two South Korean banks holding the funds.
He said he had addressed the issue on Thursday during a conversation with South Korean Foreign Minister Chung Eui-yong, and asked him to allow Iran access to its assets "as soon as possible".
US Secretary of State Antony Blinken on Thursday renewed warnings that time was running out for Iran to return to the 2015 nuclear deal between Tehran and world powers.
"The ball remains in their court, but not for long," Mr Blinken said. "There is a limited runway on that, and the runway is getting shorter."
Former president Donald Trump withdrew from the deal and reimposed sweeping sanctions, under which foreign banks have also frozen Iranian funds.
Tehran wants them removed before it returns to compliance with the nuclear deal, which it has repeatedly breached in protest against the US pressure campaign.
The administration of US President Joe Biden had been engaged in indirect talks in Vienna with Iran on returning to compliance.
Iran requested a break in talks in June due to a transition as the ultra-conservative Ebrahim Raisi took over as president.
The parties to the 2015 deal with Iran regarded it as the best way to stop Tehran from building a nuclear bomb, a goal that it has always denied.