The French foreign ministry said "significant disagreements persist" as representatives from Britain, China, France, Germany, Russia and Iran meet in Vienna in search of a breakthrough.

US President Joe Biden has indicated a willingness to rejoin the agreement, once it is certain that Iran is willing to respect its commitments, after his predecessor Donald Trump walked out of the accord.

But the expected victory of hardline judiciary chief Ebrahim Raisi in Friday's presidential election in Iran could add a new complication, even if supreme leader Ayatollah Ali Khamenei has always had the final say on the issue.

"The negotiations become more difficult as they focus on the more difficult issues. Significant disagreements persist," the French foreign ministry said in a statement.

"This means courageous decisions are needed, which will have to be taken quickly, because we all share the opinion that time is on no one's side," it added.

Raisi is regarded as the clear favourite to win after heavyweight rivals, including conservative figures, were disqualified in pre-vote vetting by an oversight body, the Guardians Council.

The French ministry was responding to comments in an Italian newspaper by UN atomic agency chief Rafael Grossi who indicated there was no prospect of a deal until a new Iranian government takes office, which may not be until August.

The landmark 2015 accord has been hanging by a thread since Trump took the United States out of the deal in 2018 and re-imposed sanctions. That led Tehran to step up its nuclear activities, which were curtailed by the deal.

Negotiators from the US are taking part indirectly in the EU-chaired discussions in Vienna.

The Iranian presidency has been held for the last eight years by Hassan Rouhani, a former chief nuclear negotiator seen as a relatively pragmatic figure on the issue.

The constitution bars him from seeking a third consecutive term.