The Islamic republic had warned it would sharply ramp up its enrichment of uranium earlier this week, after an attack on its Natanz nuclear facility that it blamed on arch-foe Israel.

That cast a shadow over the talks in Vienna aimed at rescuing a nuclear agreement between Iran and world powers that former US president Donald Trump abandoned almost three years ago.

European Union envoy Enrique Mora said Saturday that "progress has been made in a far from easy task. We need now more detailed work".

Russian ambassador to Vienna Mikhail Ulyanov added that "participants took note with satisfaction of the progress made so far and expressed determination to continue negotiations with a view to complete the process successfully as soon as possible."

The discussions involved EU officials and representatives from Britain, China, France, Germany, Russia and Iran.

The talks are aimed at determining which sanctions the United States should lift and the measures Iran has to take in order to rein in Tehran's nuclear programme.

Iran delegation head Abbas Araghchi remarked on Telegram that "a good discussion took place within the joint commission.

"It appears that a new agreement is taking shape and there is now a common final goal among all," he added.

Ali Akbar Salehi, head of the Atomic Energy Organization of Iran, confirmed Iran was now producing uranium enriched to 60 percent purity, taking the country closer to the 90-percent level required for use in a nuclear weapon.

"The enrichment of uranium to 60 percent is underway" in Natanz, he was quoted by Tasnim news agency as saying.

Iran has repeatedly insisted it is not seeking atomic weapons, but it has gradually rolled back its nuclear commitments since 2019, the year after Washington withdrew from the accord and began imposing sanctions.

The 2015 deal, known formally as the Joint Comprehensive Plan of Action (JCPOA), gave Iran relief from sanctions in return for curbs on its nuclear programme.

Iran had committed to keep enrichment to 3.67 percent, a level it raised to 20 percent in January.

Negotiations aimed at ensuring the return of the United States to the JCPOA and the lifting of sanctions resumed this week.

"We think that negotiations have reached a stage that the parties can start working on a joint text. The writing of the text can start, at least in the fields with a consensus," Araghchi said.