Iran blames its arch-enemy Israel for an incident at the Natanz nuclear facility and will take its revenge, state TV has quoted Foreign Minister Mohammad Javad Zarif as saying.
Iranian authorities described Sunday's electrical blackout as an act of "nuclear terrorism" and said Tehran reserves the right to take action against the perpetrators.
"The Zionists want to take revenge because of our progress in the way to lift sanctions," Zarif was quoted as saying.
"They have publicly said that they will not allow this. But we will take our revenge from the Zionists."
Israel's Kan public radio cited intelligence sources as saying Israel's Mossad spy agency had carried out a cyber attack at the site.
On Sunday, a spokesman for Iran's Atomic Energy Organisation said a problem with the electrical distribution grid of the Natanz site had caused an incident.
The spokesman, Behrouz Kamalvandi, said the incident caused no casualties or contamination.
Iranian media later reported Kamalvandi had an accident while visiting the Natanz site, "suffering a broken head and leg".
The reports did not elaborate on the cause of the accident.
The facility, located in the central province of Isfahan, is the centrepiece of Iran's uranium-enrichment program and is monitored by inspectors of UN nuclear watchdog the International Atomic Energy Agency.
"While condemning this despicable move, Iran emphasises the need for the international community and the International Atomic Energy Agency to deal with this nuclear terrorism and reserves the right to take action against the perpetrators," Salehi said.
Israel, which has accused Iran of seeking to build nuclear weapons, has made no official comment on the incident.
It took place a day after Tehran, which has denied it seeks atomic arms, started new advanced enrichment centrifuges at Natanz.
Kan Radio, citing the intelligence sources, said the damage at Natanz was more extensive than had been reported in Iran.
At a ceremony on Sunday with Israeli military and intelligence chiefs marking the 73rd anniversary next week of Israel's founding, Prime Minister Benjamin Netanyahu made no direct reference to Natanz.
But he said: "The fight against Iran's nuclearisation ... is a massive task".
Tehran insists its nuclear program is purely for peaceful purposes.
In July last year, a fire broke out at the Natanz facility, which Iran said was an attempt to sabotage the country's nuclear program.
In 2010, the Stuxnet computer virus, widely believed to have been developed by the United States and Israel, was discovered after it was used to attack the facility.
The incident at Natanz comes amid efforts by Tehran and Washington to revive Iran's 2015 nuclear deal with major powers after former US president Donald Trump abandoned it three years ago.
Trump reimposed sanctions that had been lifted on the Islamic Republic under the deal, and brought in many more.
The two nations laid out tough stances at indirect talks in Vienna last week on how to bring both back into full compliance with the deal.
Australian Associated Press