Iran has described a blackout at its Natanz nuclear site as an act of "nuclear terrorism," without specifying what country or entity might be behind the alleged sabotage.
Tehran had earlier said an "accident" on April 11 had caused a power failure at Natanz, one of the country's most secured underground sites.
Ali Akbar Salehi, the head of Iran's Atomic Energy Organization, urged the international community to "confront this act of nuclear terrorism."
Salehi, in a statement carried by state TV, said the attack was staged by "opponents of the country's industrial and political progress who aim to prevent development of a thriving nuclear industry."
The blackout occurred a day after Tehran launched new advanced centrifuges that more quickly enrich uranium.
Last year, a fire broke out at the Natanz nuclear facility that the government said was an attempt to sabotage its nuclear program.
The Natanz site is key to Iran's uranium enrichment program and monitored by inspectors of the International Atomic Energy Agency (IAEA), the UN nuclear watchdog.
Israel, Iran's regional archenemy, is suspected of carrying out sabotage against Iran in the past, including cyberattacks and assassinations of nuclear scientists.
Iran also blamed Israel for the killing of a scientist who began the country's military nuclear program decades earlier. Israel has not claimed any of the attacks, though Prime Minister Benjamin Netanyahu has repeatedly described Iran as the major threat faced by his country in recent weeks.
The incident at Natanz comes amid negotiations over the possible return of the United States to the 2015 nuclear deal between world powers and Tehran.
In 2018, U.S. President Donald Trump withdrew Washington from the agreement, which lifted international sanctions on Iran in exchange for curbs on Iran's nuclear program.
In reaction, Iran breached many restrictions imposed by the accord. Tehran has abandoned all the limits of its uranium stockpile. It now enriches up to 20 percent purity, a technical step away from weapons-grade levels of 90 percent.
Earlier this week, talks began in Vienna aimed at bringing the United States and Iran back into full compliance with the deal.
On April 10, Iran announced it had launched a chain of 164 IR-6 centrifuges at Natanz, injecting them with the uranium gas and beginning their rapid spinning.
Officials also began testing the IR-9 centrifuge, which they say will enrich uranium 50 times faster than Iran's first-generation centrifuges, the IR-1. The nuclear deal limits Iran to only using IR-1s for enrichment.
Iran maintains its nuclear program is solely for peaceful purposes.