An Iranian delegation met Tuesday with representatives of the remaining parties to the agreement to discuss how to bring Washington back into it and end crippling US sanctions and Iranian countermeasures.
US President Joe Biden has said he is ready to reverse the decision of his predecessor Donald Trump to withdraw from the agreement and reimpose unilateral sanctions.
But differences remain over the mechanics of the move as Tehran has since responded by suspending compliance with some of its own obligations under the deal.
The United States was not present at Tuesday's discussions because Iran has refused to meet the US delegation so long as its sanctions remain in place.
Instead, the European Union acted as an intermediary, but all sides gave a positive assessment of the opening talks.
"A new chapter has just been opened yesterday," Rouhani told a cabinet meeting Wednesday.
"If (Washington) shows it is honest and sincere, that's all we ask... I think we'll be able to negotiate in a short time, if necessary, with the (other parties to the deal)."
The talks are to resume in Vienna on Friday, a diplomat familiar with the discussions said.
The US delegation is to gather in a different hotel, with EU negotiators acting as go-betweens.
At the same time, two groups of experts -- on lifting sanctions and nuclear issues -- are working "to identify concrete measures to be taken by Washington and Tehran" to restore the deal, Russian envoy Mikhail Ulyanov said.
Washington gave an upbeat assessment of the opening session.
"We do see this as a constructive and certainly welcome step," State Department spokesman Ned Price said.
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