Iran has been engaged for a year in negotiations with France, Germany, Britain, Russia and China directly, and the United States indirectly, to revive the 2015 deal, known formally as the Joint Comprehensive Plan of Action (JCPOA).

The US unilaterally withdrew from the agreement in 2018 and reimposed sanctions on Iran, prompting Tehran to step back from nuclear commitments.

Negotiations in the Austrian capital Vienna aim to return the US to the deal, including through Washington lifting sanctions, and to ensure Tehran's full compliance with its commitments.

"Technical issues and discussions in the Vienna talks have been completed," Mohammad Eslami, head of Iran's Atomic Energy Organisation, was quoted as saying by state news agency IRNA.

"Only political issues remain," he added.

Foreign ministry spokesman Saeed Khatibzadeh said early this month that Iran will only return to Vienna to finalise an agreement, not to hold new negotiations.

The talks have been paused since March 11 after Russia demanded guarantees that Western sanctions imposed against Moscow after its February 24 invasion of Ukraine would not damage its trade with Iran.

Days later, Moscow said it had received the necessary guarantees.

Among the key remaining sticking points is Tehran's demand to delist the Revolutionary Guards, the ideological arm of Iran's military, from a US terror list.

That sanction, imposed by former US president Donald Trump after he withdrew from the nuclear agreement, is officially separate from the atomic file.

"If Iran wants sanctions-lifting that goes beyond the JCPOA, they'll need to address concerns of ours that go beyond the JCPOA," US State Department spokesman Ned Price said Monday.

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