The latest meeting is part of their regular discussions since early April, aimed at bringing the US back to the 2015 landmark agreement and Iran back into compliance with curbs on its nuclear programme in exchange for sanctions relief.

Mikhail Ulyanov, the Russian envoy at the EU-chaired talks, said Sunday's meeting would "decide on the way ahead".

"An agreement on restoration of the nuclear deal is within reach but is not finalised yet," he wrote on Twitter on Saturday.

Parties to the agreement -- Britain, China, Germany, France, Russia and Iran -- have been meeting in Vienna with indirect US participation since April to restore the deal, which promised Tehran sanctions relief in exchange for curtailing its nuclear programme.

The deal was thrown into disarray in 2018 when former US president Donald Trump withdrew and reimposed sanctions, leading Iran in turn to step up its nuclear activities from 2019 onwards.

Ultraconservative cleric Ebrahim Raisi was declared the winner of Iran's presidential election Saturday after securing just shy of 62 percent of the vote.

Negotiators have said the presidential election is not expected to influence the talks though Raisi's views are widely seen as a break from the more moderate stances of former president Hassan Rouhani.