Key players locked in talks over reviving the 2015 nuclear deal signalled that progress had been made towards ending the standoff between Iran and the US.
After days of intensive talks in the Austrian capital Vienna, Iran’s lead negotiator said discussions were on the right track, echoing a cautious sense of optimism among the EU and other signatories that an agreement could be reached.
Iran’s Deputy Foreign Minister Abbas Araghchi spoke of a “new understanding”, but said the path to a breakthrough would still be difficult.
"Too soon to predict the result," he said in a tweet on Saturday night. "Expert groups continue their hard work of clarifying important questions."
His comments injected a sense of impetus into a process that had been stalled by an attack on an Iranian nuclear plant that Iran blamed on Israel. Tehran responded by enriching uranium at levels nearer to weapons grade.
US President Joe Biden criticised that move but said it would not derail the talks – a further sign of progress in returning to an agreement struck in 2015 by former president Barack Obama.
The deal lifted sanctions on the Iranian economy in return for restrictions on Iran’s nuclear programme.
Mr Biden's predecessor Donald Trump pulled out of the deal in 2018 and embarked on a strategy of “maximum pressure” through sanctions and military action, leading to a decline in already frosty relations between the two countries.
From 2019, Iran responded to the US withdrawal by gradually increasing its atomic activity, reneging on its commitment to keep enrichment to 3.67 per cent.
The International Atomic Energy Agency confirmed on Saturday that Iran has started the process of enriching uranium to 60 per cent, a step closer to weapons grade of 80 to 90 per cent.
Iran insists that the uranium will be used for medical purposes only.
Mr Biden has pledged to return the US to the 2015 accord, but the administration is reluctant to remove sanctions all at once, something Iran insists Washington must do.
Enrique Mora, who is leading the talks on behalf of the EU, said it was key that all the parties were committed to seeing the US rejoin the accord and that it was fully implemented by both sides.
He said more detailed work was needed.
“Progress has been made in a far from easy task,” he said.
The EU, Russia and China continued to cajole the Iran and the US to revive the deal.
Mikhail Ulyanov, Russia's envoy to the International Atomic Energy Agency, said the countries would continue working into next week as they agreed to "not waste time" and reach a successful outcome "as soon as possible".