"The agenda for talks (with Enrique Mora) in Tehran is nearly finalised," foreign ministry spokesman Saeed Khatibzadeh said at a weekly press briefing.

"He will meet with Ali Bagheri, the Islamic Republic of Iran's chief nuclear negotiator."

The date of Mora's arrival in Iran's capital has not been confirmed, but local press reported he is expected on Tuesday.

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

It was agreed between Iran and the five permanent United Nations Security Council members China, Russia, the United States, United Kingdom and France, alongside Germany.

But the US unilaterally withdrew in 2018 under then-president Donald Trump and reimposed biting economic sanctions, prompting Iran to begin rolling back its own commitments.

Talks in Vienna have over the last year focused on bringing the US back into the deal and lifting its sanctions, while ensuring Tehran's full adherence to its own commitments.

Adversaries for decades, Iran and the US have been engaged in negotiations only indirectly, even while Tehran has negotiated directly with the remaining parties to the deal.

Tehran and Washington have been exchanging views through the EU's Mora.

The Vienna talks have been stalled since March.

Among the key remaining sticking points is Iran's demand that Washington delist its Islamic Revolutionary Guard Corps from a US terror list.

"Mora's trip moves the talks in the right direction," Khatibzadeh said, noting that messages are "constantly exchanged between Iran and the United States via the European Union".