Iran will begin enriching uranium to 60% purity, higher than the programme ever has before though still short of weapons grade, after an attack on its Natanz nuclear facility, an Iranian negotiator said.
The announcement marks a significant escalation after the sabotage, suspected of having been carried out by Israel.
It could result in further action by Israel, whose Prime Minister Benjamin Netanyahu has vowed never to allow Tehran to obtain a nuclear weapon, and further raise tensions across the Middle East.
Already earlier in the day, Iran’s foreign minister had warned that the weekend assault could hurt ongoing negotiations over its tattered atomic deal with world powers.
“We believe that this round of negotiations is the time for the US to present a list. I hope that I can go back to Tehran with the list of sanctions that will be lifted,” nuclear negotiator Abbas Araghchi said in Vienna, where the talks have been taking place.
“Otherwise, I don’t believe we can continue like this. Otherwise, I believe this would be a waste of time.”
Mr Araghchi said informal talks would start on Tuesday night, with a formal session beginning on Thursday.
Iran had been enriching up to 20% – even that was a short technical step to weapons-grade levels of 90%.
The International Atomic Energy Agency (IAEA) said it was aware of the media reports of Mr Araghchi’s comment, which was quoted by Iran’s state-run IRNA news agency, but had no comment at the time.
Press TV, the Iranian state television’s English-language arm, separately said that the IAEA had been informed of the move.
The broadcaster also quoted the negotiator as saying Iran would introduce another 1,000 centrifuges at Natanz, without elaborating.
Iran’s Supreme Leader Ayatollah Ali Khamenei had threatened to go to 60% enrichment in February if the country needed.
“We are determined to develop our nuclear capabilities in line with the needs of the country,” Ayatollah Khamenei said then, according to a transcript of his speech published by his website.
“For this reason, Iran’s enrichment will not be limited to 20%, and we will take whatever action is necessary for the country.”
Iran previously had said it could use uranium enriched up to 60% for nuclear-powered ships.
The Islamic Republic currently has no such ships in its navy.
Details remained scarce about the weekend attack at Natanz.
The US has insisted it had nothing to do with Sunday’s sabotage.
Instead, Israel is widely believed to have carried out the assault that damaged centrifuges, though it has not claimed it.
But earlier, Iranian foreign minister Mohammad Javad Zarif still issued a warning to Washington.
“Americans should know that neither sanctions nor sabotage actions would provide them with an instrument for talks,” Mr Zarif said in Tehran alongside visiting Russian foreign minister Sergey Lavrov.
“They should know that these actions would only make the situation difficult for them.”
Mr Zarif separately renewed his earlier warning to Israel over the sabotage, saying that if Iran determines its archenemy was behind it, “then Israel will get its response and will see what a stupid thing it has done”.
Kayhan, the hard-line Tehran newspaper, urged Iran to “walk out of the Vienna talks, suspend all nuclear commitments, retaliate against Israel and identify and dismantle the domestic infiltration network behind the sabotage”.
While Kayhan is a small-circulation newspaper, its editor-in-chief, Hossein Shariatmadari, was appointed by Supreme Leader Ayatollah Ali Khamenei and has been described as an adviser to him in the past.
Such a walkout remains unlikely as the administration of President Hassan Rouhani, whose main diplomatic achievement was the 2015 accord, hopes to get the US to rejoin it and provide desperately needed sanctions relief.
However, pressure does appear to be growing within Iran’s theocracy over how to respond to the attack.
The talks in Vienna – among Iran, world powers still in the deal and the US – are aimed at reviving America’s role in the agreement that former president Donald Trump abandoned and lifting the sanctions he imposed.
Iran, in turn, would return to the limits set by the deal and dilute its growing stockpile of uranium – some of which has been enriched up to a short step away from weapons-grade levels.
Iran insists its nuclear programme is peaceful, though the West and the International Atomic Energy Agency say Tehran had an organised military nuclear programme up until the end of 2003.
However, the deal prevents it from having enough of a uranium stockpile to be able to pursue a nuclear weapon.
Mr Rouhani met later on Tuesday with Mr Lavrov and stressed the importance of all parties returning to the deal.
Russia is a member of the nuclear deal.
“We are neither ready to accept less than that, nor are we after achieving more than that,” he said.
Meanwhile on Tuesday, Israeli broadcaster Channel 12 reported an Israeli-owned ship had been attacked in the Gulf of Oman off the coast of the United Arab Emirates near the port city of Fujairah.
The broadcaster said Israeli security officials believed it was an Iranian attack, but did not elaborate.
Iranian state media had been reporting a similar incident for hours.