The US on Tuesday criticised Iran's announcement that it would increase its uranium enrichment level to 60 per cent purity after a cyber attack at its Natanz nuclear site, which it blamed on Israel.
White House press secretary Jen Psaki said the US took the “provocative” announcement seriously.
Ms Psaki said it raised questions about “Iran’s seriousness with regard to the nuclear talks” that are set to resume this week in Vienna.
She said that the European, Russian and Chinese partners in the talks “should be unified in rejecting” the announcement.
“This step both calls into question Iran’s seriousness with regard to the nuclear talks and underscores the imperative of returning to mutual compliance with the [nuclear deal],” Ms Psaki said.
“We have been engaged in what we felt was constructive dialogue last week, even as it was indirect, in Vienna."
She said the talks were were difficult and the US expected them to be lengthy.
But Ms Psakis said the White House had not been told of any change in attendance at the meetings this week.
Iranian chief nuclear negotiator Abbas Araghchi on Tuesday said Tehran would begin 60 per cent uranium enrichment, further passing the 3.67 per cent cap laid out in the 2015 nuclear deal between Tehran and world powers.
Iran had already increased its uranium enrichment level to 20 per cent this year, continuing to add to its breaches of the accord since former US president Donald Trump withdrew from it in 2018.
The Biden administration has offered to lift the most crippling Trump-era sanctions on Iran should Tehran return to its commitments under the deal.
But Mr Araghchi also announced on Tuesday that Iran would add another 1,000 advanced centrifuges, in addition to replacing those damaged in the Natanz cyber attack.
Ms Psaki also repeated that the US was not involved in the Natanz attack.
Defence Secretary Lloyd Austin pledged on Monday that US commitment to nuclear diplomacy with Iran would continue after a visit with Prime Minister Benjamin Netanyahu in Israel.
The Biden administration’s Iran co-ordinator, Robert Malley, is expected to return to Vienna to resume talks this week.
The Iranians have refused to meet directly with the Americans, so the two sides have relayed their messages through European partners.
US and Russian diplomats offered a positive assessment after the inaugural round of talks last week, before the Natanz attack.