Talks on Iran's nuclear programme aimed at salvaging a 2015 nuclear deal were set to resume Saturday, a day after Tehran said it had started producing uranium at 60 percent purity.
The Islamic republic had declared 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.
The incidents cast a shadow over talks in Vienna aimed at rescuing the nuclear agreement between Iran and world powers that then US president Donald Trump abandoned almost three years ago.
The European Union said Saturday's talks would involve 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 to come into compliance with the accord.
The Russian ambassador to Vienna, Mikhail Ulyanov, spoke of "slow but steady progress in the negotiations on restoration of the nuclear deal" on Twitter.
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 said, quoted by Tasnim news agency.
Iran has repeatedly insisted it is not seeking an atomic bomb, but at that rate of production, it could take the Islamic republic 322 days to produce the amount of 60 percent enriched uranium needed to make one bomb, based on the International Atomic Energy Agency's (IAEA) criteria.
But this would require Iran to have a sufficient amount of 20 percent enriched uranium, which it does not have, according to the latest IAEA data.
Tehran has gradually rolled back its nuclear commitments since 2019, a 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.
Under the accord, Iran had committed to keep enrichment to 3.67 percent, though it had stepped this up 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 in Vienna.