Ali Bagheri "will return to Vienna this evening with clear instructions to pursue the negotiations with the aim of solving problems in order to reach an agreement", IRNA news agency reported.

The talks to restore the agreement known formally as the Joint Comprehensive Plan of Action involve Iran as well as Britain, China, France, Germany and Russia directly, and the United States indirectly.

On Saturday Iranian Foreign Minister Hossein Amir-Abdollahian said Tehran was studying a rough draft of a deal to revive the nuclear deal.

Iran is "seriously reviewing (the) draft of the agreement," Amir-Abdollahian said on Twitter, adding he had spoken by phone with EU foreign policy chief Josep Borrel who is coordinating the Vienna talks.

"Our red lines are made clear to western parties," the Iranian foreign minister said, adding that Tehran is "ready to immediately conclude a good deal, should they show real will".

The 2015 deal gave Iran sanctions relief in exchange for curbs on its nuclear programme, but the US unilaterally withdrew from it in 2018 under then-president Donald Trump and reimposed heavy economic sanctions.

That prompted Iran to begin rolling back on its own commitments.

The Vienna talks seek to return the US to the agreement, notably through the lifting of sanctions, and to restore Iran's compliance with its own commitments.

Tehran says it wants to verify the removal of sanctions and get guarantees that the United States will not repeat its withdrawal from the agreement.

In recent days, all sides have said the talks have reached a critical stage.