Iranian President Hassan Rouhani said on Wednesday that talks in Vienna on rescuing the nuclear deal with world powers had opened a "new chapter".
Mr Rouhani predicted that diplomats will be "able to negotiate in a short time, if necessary" after talks on a possible US return to the pact opened in the Austrian capital on Tuesday.
Washington and Tehran did not take part in direct talks, with European diplomats acting as mediators between Iranian officials and a US delegation based in a different hotel.
But Mr Rouhani told a Cabinet meeting that "a new chapter has just been opened yesterday".
If the US "shows it is honest and sincere, that's all we ask", Mr Rouhani said.
Washington also gave an upbeat assessment of the opening session.
"We do see this as a constructive and certainly welcome step," US State Department spokesman Ned Price said.
Two expert groups were convened after Tuesday's talks to establish how the US and Iran could restore compliance with the 2015 deal.
Russia also held talks on Wednesday with US envoy Robert Malley, who is leading the US delegation in Vienna.
"We had a businesslike discussion on issues related to restoration of full implementation of the [deal] by all sides," said Mikhail Ulyanov, Russia's ambassador to the UN's International Atomic Energy Agency.
The US left the pact under president Donald Trump in 2018 and re-imposed sanctions on Iran under what Washington called a "maximum pressure" campaign.
Since then, Iran has steadily flouted the restrictions it agreed to under the deal, including the amount of enriched uranium it can stockpile.
The restrictions under the pact were intended to make it harder to develop a nuclear weapon, although Tehran denies seeking to develop atomic bombs.
US President Joe Biden faces calls to tie the lifting of sanctions to the release of foreign hostages in Iran.
On the eve of the Vienna talks, a German-Iranian woman, Nahid Taghavi, was returned to solitary confinement in Tehran in what her family believes was a move intended to put pressure on Berlin, a signatory to the pact.
The talks are to resume in Vienna on Friday, a diplomat told AFP.
The US delegation led by Mr Malley is again expected to gather in a different hotel, with EU negotiators acting as go-betweens.
“There’s unity and ambition for a joint diplomatic process with two expert groups on nuclear implementation and sanctions lifting,” said Enrique Mora, deputy secretary general of the EU External Action Service, after Tuesday's talks.
“As co-ordinator, I will intensify separate contacts here in Vienna with all separate parties, including the US,” he said.
Mr Ulyanov said the two expert groups must "identify concrete measures to be taken by Washington and Tehran to restore full implementation" of the deal.
The Russian ambassador said the restoration of the nuclear deal would not happen immediately.
“It will take some time. How long? Nobody knows,” he said.
“The most important thing after today’s meeting of the joint commission is that practical work towards achieving this goal has started.”