"Working on closing the remaining gaps in the #ViennaTalks," European Union diplomat Enrique Mora tweeted ahead of his trip. "We must conclude this negotiation. Much is at stake."

Iran has been engaged in efforts to revive the so-called Joint Comprehensive Plan of Action (JCPOA) with Britain, China, France, Germany and Russia directly, and the United States indirectly since April 2021.

Mora, who coordinates indirect talks between Iran and the US, arrived in Tehran late Saturday and on Sunday met Foreign Minister Hossein Amir-Abdollahian, state news agency IRNA reported.

The report gave no details on the meeting or earlier discussions between Mora and Iran's chief negotiator Ali Bagheri.

"Bagheri stressed that an agreement can be reached if the American side is realistic," IRNA said.

Mora's visit comes as EU foreign policy chief Josep Borrell voiced hope for a renewed accord within "days".

"We are very close but there are still some issues pending," Borrell told reporters on the sidelines of the Doha Forum in Qatar.

"I cannot tell you when or how, but it is a matter of days."

Parties have signalled for weeks that the negotiations are close to an agreement, but that "political decisions" are required from Tehran and Washington.

The 2015 deal gave Iran much-needed sanctions relief in exchange for curbs on its nuclear programme that would guarantee it could not develop a nuclear weapon -- something it has always denied wanting to do.

The deal fell apart in 2018 after the US under then-president Donald Trump unilaterally withdrew from the accord and reimposed crippling sanctions on Iran.

Tehran retaliated by rolling back most of its commitments from 2019.

Iran's top diplomat has said one of the key outstanding issues is removing the Islamic Revolutionary Guard Corps (IRGC) from a US terrorist list.

Senior Guard officials have suggested the issue be dropped if a deal ensured Iran's "national interests", Amir-Abdollahian said Saturday.

The US special envoy for Iran, Robert Malley, said in Qatar on Sunday that Washington would maintain sanctions on the Guards, the ideological arm of Iran's military.

"The IRGC will remain sanctioned under US law and our perception of the IRGC will remain," Malley said.