European Union envoy Enrique Mora, who is the coordinator of Vienna talks to revive Iran’s nuclear deal, says the success of negotiations is still uncertain.

Mora made the remarks in a tweet on Sunday following a meeting of the working group on the removal of US sanctions on Tehran.

Saying that more meetings were scheduled for the day with experts from the participating sides, Mora emphasized that uncertainty about the result is “logical in such a complex negotiation”.

Mora also appreciated the commitment of the negotiating teams to reaching a final agreement.

Also on Sunday, Russia’s top negotiator Mikhail Ulyanov twitted about a meeting of the working group on implementation, which is set to coordinate the sequencing of steps on Iran’s nuclear activities and lifting of sanctions.

On Friday, French Foreign Minister Jean-Yves Le Drian said it is “vital” that Vienna talks “succeed” because if negotiations continue with the current speed there will be nothing left to negotiate about.

He sounded less optimistic than EU foreign policy chief Josep Borrell who said that a renewed deal with Iran is still "possible in the coming weeks” as the talks are advancing in a "better atmosphere".

Earlier on Friday, chief negotiators of Iran and the three European participants in the talks returned to their capitals to hold consultations and update their respective governments.