Iran on Monday accused Israel of sabotaging its Natanz underground nuclear facility in an attempt to derail talks to revive Tehran's nuclear deal with world powers.
Foreign Minister Javad Zarif said Israel wanted revenge after negotiations took place in Vienna to bring the US back to the 2015 agreement and remove sanctions against Iran.
He said Iran would avenge the attack, which damaged centrifuges used to enrich uranium.
Iran's Information Ministry said a perpetrator was identified and efforts were under way to arrest him, the local Tasnim news agency reported.
Israel has not officially commented on the incident, but Israeli media, quoting intelligence officials, said a cyber attack by Mossad was responsible for the blackout.
"Natanz will be built stronger than in the past using more advanced machines and if they think our hand in negotiation is weakened, incidentally this act will make our position stronger," said Mr Zarif during a meeting with Iran’s National Security Commission, state news agency IRNA reported.
The Foreign Ministry spokesman also said the incident at Natanz was aimed at disrupting the Vienna talks.
"Iran's response is revenge against the regime in a due time and place," said Saeed Khatibzadeh.
He confirmed there was no human or environmental damage, but said it could have led to a "catastrophe".
Mr Khatibzadeh said all the centrifuges that went out of circuit were IR-1, the only type of centrifuge that Iran is permitted to use for enrichment under the terms of the nuclear deal.
The affected centrifuges will be replaced with advanced models, he said.
At the weekend, Iran said it started using advanced uranium enrichment centrifuges, a breach of the 2015 nuclear deal.
The nuclear enrichment site was affected by a power disruption only hours after starting up the advanced centrifuges, state TV reported.
Iran's nuclear chief Ali Akbar Salihi said the plant was still operating on Monday using an emergency electrical grid. He added that some decommissioned machines would be re-evaluated for possible use.
Vienna talks
Countries involved in the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), began negotiations in Vienna last week.
US President Joe Biden said he was willing to re-commit to the deal after former president Donald Trump withdrew the US from the pact in 2018 and imposed sanctions on Iran. Israeli Prime Minister Benjamin Netanyahu has promised to block a deal that allowed Iran to build its nuclear arsenal.
The White House said on Monday that it was aware of the reports of the incident at Natanz but the US had nothing to do with it and would not speculate about the causes of the attack.
Germany, a participant in the Vienna talks, said the incident at Natanz put an obstacle in the way of negotiations.
"What we are hearing currently out of Tehran is not a positive contribution, particularly the development in Natanz," said German Foreign Minister Heiko Maas before talks continued on Tuesday.
Mr Maas said the negotiations in Vienna "will not be easy but until now, there has been a constructive spirit" shown by participants.
Mr Trump's withdrawal from the nuclear deal prompted Iran to harden its stance and advance its enrichment programme beyond the limits set by the 2015 deal.
The UN's nuclear watchdog has noted several breaches by Iran as it accelerates its programme, including increasing its uranium stockpile tenfold over what is permitted in the deal.
In January, Iran began enriching uranium at Fordow, an underground site that was built inside a mountain, possibly to withstand any aerial bombardment.
Under the deal, Tehran is not allowed to enrich any uranium at Fordow.
Iran claims 'nuclear terrorism'
Iranian authorities described the incident on Sunday as an act of "nuclear terrorism" and said Tehran reserved the right to take action against the perpetrators.
"The Zionists want to take revenge because of our progress in the way to lift sanctions ... they have publicly said that they will not allow this. But we will take our revenge from the Zionists," Mr Zarif said.
Iran’s nuclear chief described the incident at Natanz as “sabotage”.
The incident on Sunday involved the electrical distribution grid at the site and is the latest in a series of problems to befall Iran’s most secure nuclear development site.
The Natanz plant has been the target of several attacks. It was first targeted by a major cyber attack about a decade ago. The Stuxnet virus infected control units for centrifuges at Natanz, causing the sensitive devices to spin out of control and destroy themselves.
The US and Israel were believed to have been behind that attack, although neither country has ever acknowledged responsibility.
Another attack targeted Natanz in July, setting Iran's nuclear plans back by months after a fire at an advanced centrifuge assembly plant.
Afterwards, Iran said it would rebuild the site deep inside a nearby mountain, and satellite photos showed that work was under way. Suspicion fell on Israel for the blast.
Possible Iranian response
Sima Shine, head of the Iran programme at Israel's Institute for National Security Studies, said Tehran's poor economic situation and the coronavirus pandemic may hamper its ability to retaliate following the Natanz incident.
"It's not that Iran is in its best situation to just decide what to do, I think they are sitting now and planning what they can do. But do they have all the options on the table? I'm not sure," she said at a press briefing.
Iran could reduce its cooperation with inspectors from the International Atomic Energy Agency, or attempt to attack Israeli civilian infrastructure, she said.
"If it's direct, vis a vis Israel, will there be any casualties? Because if that happens, probably Israel will retaliate, this can start a slippery slope of escalation between the two countries," said Ms Shine, who previously served as head of research and evaluation at Israel's Mossad intelligence agency.
