The EU official leading talks to revive Iran's nuclear deal said on Wednesday he was "quite sure" an agreement would be reached as the negotiations adjourned for a week.
The 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 talks with the United States on how to resume compliance with the deal, which former President Donald Trump abandoned in 2018, prompting Tehran to begin violating its terms about a year later.
"I am quite sure that there will be a final agreement... I think we are on the right track and we will get an agreement," 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.
Russia's envoy, Mikhail Ulyanov, said on Twitter that participants felt there had been good progress after the latest round and that a deal was "within reach". Ulyanov said he hoped that next week would be the final round.
Asked if he was saying there would be a deal in the next round, Mora said: "I cannot venture such a prediction. What I can venture is that there will be an agreement, yeah, sure."
Ahead of the meeting, Iran's top nuclear negotiator Abbas Araqchi told Iranian state TV there had been "good" progress in the talks but "several key issues needed further discussions".
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.
"An agreement is shaping up. Now a common understanding on what still needed for US return to #JCPOA, lifting of related sanctions and the resumption of nuclear commitments by Iran," Mora said on Twitter, referring to the Joint Comprehensive Plan of Action.
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 Iran was continuing to negotiate with the IAEA on extending that agreement.