Iran is blaming Israel for a blackout at the country's underground Natanz nuclear facility.
Iranian Foreign Ministry spokesman Saeed Khatibzadeh said uranium enrichment centrifuges were damaged, and that Iran would take unspecified revenge against Israel.
Iran's nuclear chief Ali Akbar Salehi called Sunday's incident an act of "nuclear terrorism."
"While condemning this desperate move, the Islamic Republic of Iran emphasizes the need for a confrontation by the international bodies and the (International Atomic Energy Agency) against this nuclear terrorism," he said, according to state TV.
The International Atomic Energy Agency, the United Nations body that monitors Iran's atomic program, said Sunday that it was aware of the situation and following developments, but did not elaborate.
Citing two intelligence officers who said they had been briefed on the damage, the New York Times reported that the incident was a severe blow to Iran's ability to enrich uranium, and it could take up to nine months to restore Natanz.
The Times said U.S. and Israeli intelligence officials said there had been an Israeli role in the blackout. Multiple Israeli media outlets reported Sunday that Israel could have been behind the blackout, which may have been a cyberattack.
The blackout comes as the United States and Iran are set to have a second round of indirect talks this week to try to revive the 2015 nuclear deal Iran reached with world powers. The deal is formally known as the Joint Comprehensive Plan of Action.
Iran has maintained that all U.S. sanctions against it must be lifted, and the U.S. warned that such a demand may lead to an impasse. U.S. President Joe Biden has pledged to return to the JCPOA if Iran first resumes full compliance.
Then-U.S. President Donald Trump pulled the U.S. out of the agreement in 2018 and a reimposition of American sanctions on Iran became a major irritant in relations between the United States and Europe. Iran in turn began taking steps away from its commitments as it sought sanctions relief, including holding larger stockpiles of enriched uranium and enriching the material to higher levels.
The Natanz facility has been subject to attacks in the past, including the Stuxnet cyberattacks nearly a decade ago, which was widely blamed on the United States and Israel. Natanz also suffered a mysterious explosion of its centrifuge assembly plant last July, which Iranian authorities described as sabotage.
The U.S. and Israel have not commented on the latest blackout.
The incident came one day after Iranian President Hassan Rouhani oversaw on live television the launch at Natanz of advanced centrifuges to enrich uranium, a key component for nuclear weapons, while reiterating his country's commitment to nuclear non-proliferation.
Rouhani reiterated claims that the country's nuclear activities are "peaceful and for non-military purposes."