South Korea’s prime minister arrived in Tehran for talks over $7 billion of Iranian funds trapped in his country by U.S. sanctions, with a successful outcome likely to be seen as a confidence-building measure between the Islamic Republic and world powers. Chung Sye-Kyun’s two-day trip, the first by a South Korean premier in 44 years and the highest-ranking visit by a Korean official since 2016, comes two days after Iran said it had freed a South Korean-flagged tanker seized in January.

Iranian officials linked the vessel’s capture to the funds, which have been inaccessible since then-President Donald Trump withdrew from the 2015 nuclear deal and reimposed sanctions on Tehran.

The U.S. and Iran, with the help of other signatories, are trying to find a way for the Biden administration to rejoin the accord and lift the penalties at talks in Vienna. Seoul is only expected to release some or all of the Iranian billions after a nod from Washington.

Iran escalated its nuclear activity after the U.S. abandoned the 2015 deal, fueling regional tensions, and on Saturday started injecting uranium gas into new centrifuge machines at its Natanz facility. The plant was hit by a so far unexplained power outage on Sunday that raised suspicions of sabotage.

Iranian officials have said they’ve been unable to access the country’s oil revenues from exports to South Korea as no bank has been willing to handle the transactions for fear of contravening the sanctions. They have accused Seoul of bending before U.S. pressure in defiance of official exemptions for humanitarian goods.