The negotiations also aim to lift sanctions against Iran and bring it back into compliance with nuclear commitments it made to world powers as part of the deal.
Iran has condemned the rebuke as “unconstructive” and announced on Wednesday it had disconnected some IAEA cameras monitoring its nuclear sites.
IAEA chief Rafael Grossi said Thursday his agency had been informed that 27 cameras were being removed, leaving about 40 still in place.
“So this of course poses a serious challenge to our ability to continue working there,” Grossi told reporters, urging Iran to engage with him “immediately”.
He said if a solution was not found within three to four weeks, “a fatal blow” will have been dealt to negotiations.
Wednesday’s motion — approved by 30 of the 35 members of the IAEA board of governors, with only Russia and China voting against — was the first to criticise Iran since June 2020.
The resolution — submitted by the United States, Britain, France and Germany — came after the IAEA said Iran continued to fail to explain adequately the previous discovery of traces of enriched uranium at three sites which Tehran had not declared as having hosted nuclear activities.
US Secretary of State Antony Blinken warned Thursday that Iran’s actions will undermine attempts to restore the 2015 deal.
“The only outcome of such a path will be a deepening nuclear crisis and further economic and political isolation for Iran,” Blinken said in a statement.
Britain, France and Germany in a joint statement urged Iran to “cease its nuclear escalation, and urgently conclude the deal currently on the table… while it is still possible”.
But Iran’s President Ebrahim Raisi, an ultraconservative who was elected last year, said the Islamic republic would not be deterred.
“We won’t back down, not even a step from our position,” state news agency IRNA quoted him as saying on Thursday.
Iran has repeatedly denied any ambition to develop a nuclear weapons capability.
It had already responded angrily to Grossi’s decision to visit its arch-foe Israel ahead of the board of governors meeting. It has also accused the UN watchdog of relying too much on “fabricated” Israeli intelligence reports.
Israeli Prime Minister Naftali Bennett welcomed the censure of Iran before he headed to the United Arab Emirates, a fellow Iran critic, for a previously unannounced visit on Thursday.
Eric Brewer, an analyst at the non-profit Nuclear Threat Initiative, told AFP that Iran’s removal of the cameras “will certainly put added pressure” on the talks to revive the 2015 agreement “to force a decision one way or the other”.
The landmark agreement set limits to Iran’s nuclear activities in return for relief from international sanctions. But it has been in disarray since Trump unilaterally withdrew and reimposed crippling sanctions.
In response, Iran began rolling back on its own commitments under the deal.
Western capitals have expressed mounting concern over how far Iran has gone in resuming nuclear activities since the US began reimposing sanctions.
Iran has built up large stockpiles of enriched uranium, some of it enriched to levels far higher than those needed for nuclear power generation.
The foreign ministry in Tehran said on Wednesday that besides deactivating the cameras in response to the IAEA censure motion, Iran has also installed additional advanced centrifuges for uranium enrichment.
The IAEA head said on Monday it would be “a matter of just a few weeks” before Iran could obtain sufficient material for a nuclear weapon, if it continues to develop its programme.
Several people in a Tehran shopping area said on Thursday that they wished their country would be more amenable to the IAEA.
“We call on the authorities to cooperate more so that the problems do not get worse,” Ebrahim Ahmadpour, a 60-year-old private sector employee, told AFP.