The Biden administration is holding indirect discussions with Iran on a possible prisoner exchange in a bid to secure the release of American citizens imprisoned in Iran, with Qatar and the United Kingdom playing an intermediary role in the talks, according to four sources familiar with the matter.

The negotiations have made progress, but it remains unclear if a final agreement will be reached, the sources said.

The push by the White House comes after months of negotiations aimed at reviving a 2015 nuclear agreement have stalled and as the administration has announced new sanctions against Tehran over its delivery of drones to Russia.

The two sides are exploring a formula that has been discussed previously, dating to 2021, that could include a possible prisoner exchange and the release of billions of dollars in funds in South Korea banks currently blocked by U.S. sanctions, three sources with knowledge of the talks said. 

The proposed formula would allow Iran access to the funds but only for the purchase of food, medicine or other humanitarian purposes, in accordance with existing U.S. sanctions against Iran.

In the discussions, U.S. and Iranian diplomats have explored possible arrangements for how to transfer the frozen funds, with a third country such as Qatar possibly overseeing the transfer, the sources said.

This month, Qatari Foreign Minister Mohammed bin Abdulrahman Al-Thani met his Iranian counterpart, Hossein Amir-Abdollahian. During the meeting, the Qatari delegation “conveyed messages from the U.S. to the Iranians which included points on the prisoner release,” said a source with knowledge of the talks.

In an interview this month with NPR, the Iranian foreign minister, Amir-Abdollahian, said discussions were underway on a possible prisoner swap with third parties helping to relay messages. He said a “U.K. official” was acting as a “representative” for the U.S. in the talks.

“The representative in question was in Iran in the past weeks, and we updated the agreement that we had back in March,” he said. “We’re ready to exchange our prisoners, but there are technical steps that need to be taken by the Americans. We are awaiting the technical steps to be taken.”

Asked about possible indirect negotiations, the Biden administration has suggested it has methods for relaying messages to Iran.