The next few days are widely seen as a crunch point for the negotiations, the latest round of which started in late November in Vienna.

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

She added that they were "ready to return soon".

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.

The EU has been chairing the talks and the bloc's foreign policy chief Josep Borrell said at a press conference on Friday he "hopes to have results this weekend", while stressing that there was "still work ongoing".

The 2015 deal, also known as the Joint Comprehensive Plan of Action (JCPOA) was aimed at guaranteeing that Iran's nuclear programme could not be used to develop a nuclear weapon -- something Tehran has always denied wanting to do.

It 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.

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.

Earlier this week Lavrov cancelled a planned trip to Geneva, with Russia's mission there saying this was "due to an unprecedented ban on his flight in the airspace of a number of EU countries that have imposed anti-Russian sanctions".