Iran's chief nuclear negotiator Ali Bagheri-Kani arrived in Moscow on an unannounced visit after the talks between Tehran and Washington ended in Doha.
The Russian Permanent Mission to the International Organizations in Vienna announced the visit on its Twitter account. However, Iranian media outlets did not report the news.
Bagheri-Kani met with his Russian counterpart, Deputy Foreign Minister Sergei Ryabkov. The meeting was attended by Moscow's chief negotiator Mikhail Ulyanov, who described the meeting as a "very professional exchange of views."
Ulyanov tweeted: "It was a very professional exchange of views on the current situation around the JCPOA and prospects of the Vienna Talks. My assessment: despite all the difficulties, the nuclear deal still can be restored."
He called on the US to "demonstrate greater flexibility."
Earlier in the week, Bagheri-Kani met in Doha with the EU coordinator, Enrique Mora, who chaired year-long talks in Vienna to revive the 2015 agreement.
On Friday, the UN Under-Secretary-General for Political Affairs, Rosemary DiCarlo, said that achieving the "landmark" Joint Comprehensive Plan of Action (JCPOA) took determined diplomacy, adding that restoring it will require additional effort and patience.
DiCarlo called Washington and Tehran to "quickly mobilize in this same spirit and commitment to resume cooperation under the JCPOA."
