"We have made substantial progress over the last 10 days but there are still things to be worked on and we will reconvene next week and we will continue working," Enrique Mora, who is coordinating indirect talks between Iran and the United States , told reporters at the end of a fourth round of negotiations in Vienna.
Talks resumed in Vienna on May 7 with the remaining parties to the deal - Iran, Russia, China, France, Britain and Germany - meeting in the basement of a luxury hotel, and the United States based in another hotel across the street.
Iran has refused to hold direct meetings with the United States on how to resume compliance with the deal, which former President Donald Trump abandoned in 2018, prompting Iran to begin violating its terms about a year later.
"I am quite sure that there will be a final agreement," Mora said. "I think we are on the right track and we will get an agreement."
Asked if he was saying there would be a deal in the next round: "I cannot venture such a prediction. What I can venture is that there will be an agreement, yeah, sure."
The crux of the original agreement was that Iran committed to rein in its nuclear programme to make it harder to obtain the fissile material for a nuclear weapon in return for relief from U.S., EU and U.N. sanctions.
Officials have said they hope to reach a deal by May 21, when an agreement between Tehran and the IAEA, the U.N. nuclear watchdog, on continued monitoring of some Iranian nuclear activities is due to expire.
Mora said he Iran was continuing to negotiate with the IAEA on extending that agreement.