The International Atomic Energy Agency (IAEA) said on Thursday that Iran had begun removing 27 of the UN watchdog's monitoring equipment, which had been installed under the 2015 nuclear deal.
The Joint Comprehensive Plan of Action, as the 2015 agreement is called, was aimed at preventing Iran from developing nuclear weapons.
The IAEA revelations comes as tensions have mounted with Iran over its nuclear program, with Tehran enriching uranium closer than ever to weapons-grade levels.
On Wednesday, the IAEA board of governors censured Tehran for failing to provide "credible information'' over man-made nuclear material found at three undeclared sites in the country.
The censure was submitted by the UK, France, Germany and the US to the International Atomic Energy Agency (IAEA) and was backed by 30 countries on the watchdog's board, with only China and Russia opposing.
Iran criticized the IAEA resolution, describing it on Thursday as "hasty" and "unbalanced."
A 'fatal blow' to 2015 deal
IAEA chief Rafael Grossi said during a press conference that the removal of the cameras posed "a serious challenge" to the UN's ability to monitor Iran's nuclear program.
It would mean "less transparency, more doubt, more uncertainty" surrounding Iran's nuclear program, Grossi added.
"We are in a very tense situation," Grossi stressed.
If Iran refused to reinstall some of the monitoring equipment, Grossi said it would be "a fatal blow" in ongoing talks to revive the 2015 deal.
US urges Iran to cooperate with IAEA
US Secretary of State Antony Blinken urged Iran on Thursday to cooperate with the IAEA.
Blinken echoed Grossi's remark, saying Iran's actions threatened the possible restoration of the 2015 nuclear deal.
"The only outcome of such a path will be a deepening nuclear crisis and further economic and political isolation for Iran," he said.
Moreover, Blinken said that negotiations on the Iran nuclear deal can only conclude if Tehran drops its extraneous demands.
Under former President Donald Trump, the United States unilaterally pulled out of the nuclear deal in 2018 and began reimposing sanctions against Iran. Talks in Vienna on reviving the agreement have stalled since April.