"We are close. E3 negotiators leaving Vienna briefly to update ministers on state of play," British diplomat Stephanie Al-Qaq said on Twitter, referring to negotiators from Britain, France and Germany.

Along with counterparts from China, Iran and Russia they have been taking part in the latest round of talks in the Austrian capital since late November.

The US has been participating indirectly.

Also on Friday, Iran's Foreign Minister Hossein Amir-Abdollahian said he was prepared to travel to Vienna if a deal is reached to revive the accord.

Austrian Chancellor Karl Nehammer told reporters that "we have received very positive signals that (Russian) Foreign Minister (Sergei) Lavrov will take part" in an expected meeting of foreign ministers in the event of an agreement being reached.

The 2015 deal began unravelling when former US president Donald Trump withdrew from it in 2018 and re-imposed sanctions, prompting Iran to start disregarding the limits on its nuclear activity laid down in the agreement.