Iran announced on Saturday it has started up advanced uranium enrichment centrifuges in a breach of its undertakings under a troubled 2015 nuclear deal, days after the start of talks on rescuing the accord.
The news follows revelations from the International Atomic Energy Agency (IAEA) that Iran was also involved in a separate breach of the deal, involving scrap uranium fuel.
The United States had said on Friday that it had offered "very serious" ideas on reviving the accord but was waiting for Tehran to reciprocate, something Saturday's move signally failed to do.
President Hassan Rouhani inaugurated the three cascades of 164 IR-6 centrifuges, 30 IR-5 and another 30 IR-6 devices at Iran's Natanz uranium enrichment plant in a ceremony broadcast by state television.
The television aired no images of the injection of uranium hexafluoride gas into the cascades, but broadcast a link with engineers at the plant who said they had started the process.
Iran's latest move to step up uranium enrichment follows an opening round of talks in Vienna on Tuesday, with representatives of the remaining parties to the nuclear deal on bringing the United States back into it.
The Vienna talks are focused not only on lifting crippling economic sanctions imposed by Donald Trump's administration, but also aim to bring Iran back into compliance with the 2015 nuclear deal, after it responded by suspending several of its own commitments.
All sides said the talks, in which Washington is not participating directly but has the European Union as intermediary, had got off to a good start.
Iran has demanded that the United States first lift all sanctions imposed by Mr Trump, which include a sweeping ban on its oil exports, before it falls back in line with obligations it suspended.
The "US – which caused this crisis – should return to full compliance first," Foreign Minister Mohammad Javad Zarif tweeted, adding that "Iran will reciprocate following rapid verification."
Washington has demanded movement from Tehran in return.
"The United States team put forward a very serious idea and demonstrated a seriousness of purpose on coming back into compliance if Iran comes back into compliance," a US official told reporters as talks broke for the weekend.
But the official said the United States was waiting for its efforts to be "reciprocated" by Iran.
The US official indicated that the major stumbling block in the initial talks was not the order of compliance but rather which sanctions were under discussion, as Iran is demanding an end to all US restrictions.
The deal, formally known as the Joint Comprehensive Plan of Action, covers only nuclear sanctions and not US measures taken in response to human rights or other concerns, the official said.
The centrifuges started up on Saturday allow uranium to be enriched more quickly and in greater amounts than the Iran's first generation devices, which are the only ones that the 2015 deal allows it to use.
Scrap uranium revelations
President Rouhani's announcement on accelerated enrichment follows revelations from the UN atomic watchdog on a separate breach of the deal, on Friday.
The latest IAEA report will likely raise tensions with Western powers.
The IAEA avoids saying Iran has breached the deal.
However, it generally only issues such ad hoc reports to member states in the event of a breach.
Two diplomats told Reuters what the report described amounted to a fresh breach.
After the deal was reached in 2015, the parties to it defined what should count towards the stockpile, and excluded items such as scrap fuel plates with uranium enriched to near 20 per cent fissile purity, which were deemed "unrecoverable."
Friday's report, however, said Iran had recovered some of that material.
""On 7 April 2021, the Agency verified at the Fuel Plate Fabrication Plant at Esfahan that Iran had dissolved six unirradiated scrap fuel plates for the TRR (Tehran Research Reactor) containing 0.43 kg of uranium enriched up to 20% U-235," the report said.
After talks among the remaining parties to the deal wrapped up on Friday, France's Foreign Ministry said a "positive" first week of negotiations should not be undermined by new Iranian provocations.
"In this context, it is all the more important that Iran refrain from any further violation of its nuclear commitments that could undermine the current dynamic," Foreign Ministry spokeswoman Agnes von der Muhll said.
David Albright, a former UN weapons inspector who advocates a tough policy stance on Iran, said the latest breach also raises questions about what major powers excluded from the enriched uranium stockpile.
"Looking back, exempting this near 20 per cent enriched uranium scrap was probably not a good idea," he said, explaining what scrap means in this case:
"When enriched uranium is made into fuel plates, some does not get used, somewhat like batter for a cake."