The Natanz nuclear facility in Iran mysteriously lost power on Sunday in what Iranian officials called an act of sabotage, a development that comes amid new negotiations aimed at salvaging the nuclear deal repudiated by the Trump administration three years ago.

Ali Akbar Salehi, the head of Iran’s Atomic Energy Organization, described the blackout as an act of “nuclear terrorism” and said the international community must confront this threat.

“The action this morning against the Natanz enrichment site shows the defeat of those who oppose our country’s nuclear and political development and the significant gains of our nuclear industry,” Mr. Salehi said, according to the Iranian news media. “The incident shows the failure of those who oppose Iran negotiating for sanctions relief.”

Iranian officials did not specify whom they blamed, but suspicions immediately fell on Israel, which has sabotaged Iran’s nuclear work previously with tactics ranging from cyberattacks to outright assassinations. A U.S. official confirmed an Israeli role in the power outage at Natanz on Sunday but said details were still very sketchy.

Israel is believed to have orchestrated the killings of several Iranian nuclear scientists in recent years — including a deadly ambush of a key developer of its nuclear program last November. Israel, as a matter of policy, neither confirms nor denies such actions.

The blackout hit Natanz barely a week after the United States and Iran, in their first significant diplomacy under the Biden administration, participated in new talks aimed at reviving the 2015 nuclear agreement abandoned by President Donald J. Trump. Mr. Biden has said he wants to resurrect the accord, known as the Joint Comprehensive Plan of Action, or J.C.P. O.A, through the talks, which began in Vienna last Monday and are set to resume this week.

It was not immediately clear if the blackout would affect the resumption of the talks. But Iran now faces a complicated calculation on how to respond, especially if it concludes that Israel was responsible.

“Tehran faces an extremely tricky balance,” said Henry Rome, an Iran analyst at the Eurasia Group, a political risk consultancy. “It will feel compelled to retaliate in order to signal to Israel that attacks are not cost free.”

At the same time, Mr. Rome said, “Iran also needs to ensure that such a retaliation does not make it politically impossible for the West to continue pushing forward with J.C.P.O.A. re-entry.”

Power was cut across the Natanz facility, Behrouz Kamalvandi, a civilian nuclear program spokesman, told Iranian state television. He said there had been no casualties or damage.

Malek Shariati Niasar, an Iranian lawmaker who serves as a spokesman for the Parliament’s energy committee, said on Twitter that the outage was “very suspicious,” He raised the possibility of “sabotage and infiltration.”

The blackout came less than year after a mysterious fire ravaged part of the Natanz facility, about 155 miles south of Tehran, the capital. Iranian officials initially played down the effect of the fire but later admitted it had caused extensive damage.

Further raising suspicions, the blackout came a day after Iranian officials lauded the inauguration of new centrifuges housed at a site constructed following the Natanz fire. It was not clear from the announcements on Sunday whether those new centrifuges were adversely affected by the power failure.

Some Iranian experts dismissed the idea that a cyberattack could have caused power outages at Natanz. The complex there has its own power grid, multiple backup systems and layers of security protection to prevent a cyberattack that would abruptly shut down its system.

“It’s hard to imagine that it was a cyberattack,” said Ali Vaez, the Iran project director at the International Crisis Group. “The likely scenario is that it either targeted the facility indirectly or through physical infiltration.”

Israel welcomed President Trump’s repudiation of the nuclear deal three years ago, and it has expressed deep concern about Mr. Biden’s intentions to restore it and rescind many of the sanctions Mr. Trump imposed on Iran.

While there is no direct dialogue between Iran and the United States at the talks in Vienna, the other participants in the agreement — Britain, China, France, Germany and Russia, under the chairmanship of the European Union — are engaging in a form of shuttle diplomacy.

One working group is focusing on how to lift economic sanctions imposed by the Trump administration, while another is looking at how Iran can return to the terms that set limits on enriched uranium and the centrifuges needed to produce it.

Iran has said that its nuclear ambitions are peaceful, and that while it intends to steadily resume nuclear activities prohibited under the deal, it could easily reverse course if the sanctions were rescinded.

On Saturday, President Hassan Rouhani of Iran celebrated the inauguration of the new centrifuges, which shorten the time needed to enrich uranium, the fuel for nuclear bombs.

But Mr. Rouhani also insisted that Iran’s efforts were not intended to produce weapons.

“If the West looks at the morals and beliefs that exist in our country, they will find that they should not be worried and sensitive about our nuclear technology,” Mr. Rouhani said in remarks reported by Iran’s Mehr News Agency.

The new centrifuges were inaugurated on what Iran calls its National Nuclear Day, an annual event to showcase the country’s advances in nuclear technology despite its economic isolation. The celebrations even included the debut of a music video that featured singing white-robed scientists standing beside centrifuges and holding photos of colleagues who had been assassinated.

Word of the Natanz outage came as Lloyd Austin, the U.S. defense secretary, was in Israel on Sunday for talks with Prime Minister Benjamin Netanyahu and the country’s defense minister, Benny Gantz.

At the meeting, Mr. Gantz said, “We will work closely with our American allies, to ensure that any new agreement with Iran will secure the vital interests of the world and the United States, prevent a dangerous arms race in our region and protect the State of Israel.”

The United States and Israel have a history of covert collaboration, dating to the administration of President George W. Bush, in disrupting Iran’s progress toward developing nuclear energy.

The best-known operation under this collaboration, which was code-named “Olympic Games,” was a cyberattack disclosed during the Obama administration that disabled nearly 1,000 centrifuges at Natanz and that was believed to have set back Iran’s enrichment activities by many months.

David E. Sanger, Eric Schmitt and Gerry Mullany contributed reporting.

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