After Nuclear Site Blackout, Thunder From Iran, and Silence From U.S.

JERUSALEM — The last time the centrifuges crashed at Iran’s underground nuclear fuel-production center at Natanz, more than a decade ago, the sabotage was the result of a joint Israeli-American cyberattack intended to slow Tehran’s progress toward nuclear weapons and force a diplomatic negotiation.

When they crashed again this weekend, the White House asserted that the United States had no involvement.

The operation raised the question of whether Israel was acting on its own to strike Iran and undermine American diplomacy as the Biden administration seeks to reconstitute a nuclear agreement. Or, alternatively, whether Israel was operating in concert with American interests, carrying out dirty work that would weaken Iran’s negotiating position in the talks.

The White House was saying almost nothing in public on Monday about the apparent explosion inside Iran’s Natanz facility, below more than 20 feet of reinforced concrete, which destroyed the power supply that keeps the centrifuges spinning at supersonic speeds, enriching uranium.

“The U.S. was not involved in any manner,” the White House spokeswoman, Jen Psaki, said Monday. “We have nothing to add on speculation about the causes or the impacts.”

White House officials did not comment on whether the United States had been given advance notice of the attack.

Defense Secretary Lloyd J. Austin III, who landed in Israel on Sunday, the morning the attack took place, held two press briefings before he left Israel on Monday and never once uttered the word Iran.

White House and State Department officials said they had no idea whether the Iranians would show up in Vienna again on Wednesday, when the talks were scheduled to resume.

In Tehran, lawmakers asked Foreign Minister Mohammad Javad Zarif to suspend the talks, saying that Iran should not be engaged in negotiations when it is under attack.

“Talks under pressure have no meaning,” said Abbas Moghtadaie, the deputy chairman of Parliament’s national security and foreign policy committee, said in a Clubhouse talk on Monday. “This was a message we conveyed very clearly today.”

The Biden administration is seeking to revive an agreement, scuttled by President Donald J. Trump three years ago, in which Iran accepted limits on its nuclear program in exchange for the lifting of sanctions. Israel’s prime minister, Benjamin Netanyahu, opposed the original agreement and has made no secret of his opposition to resurrecting it.

Mr. Zarif, in a statement broadcast by Iranian state television, said that Israel wanted “to take revenge because of our progress in the way to lift sanctions.”

“But we will take our revenge on the Zionists,” he continued.

His comments highlighted the risk of escalation in a yearslong shadow war between Iran and Israel, one that is taking place in the deserts of Natanz, along the shipping routes of the Persian Gulf and in the leafy suburbs of Tehran, where Mohsen Fakhrizadeh, the leader of what American intelligence officials say was Iran’s secret nuclear weapons program, was killed in December by a remote-controlled gun as he drove to his weekend house.

For the Iranians, the attack this weekend was another humiliating indication that its program had been penetrated by spies and saboteurs, who have carried out a series of brazen attacks. While Israel usually stays silent when attacks like this happen, Israeli news outlets, citing intelligence sources, attributed this one to the Mossad, the Israeli spy agency.

An intelligence official who asked not to be identified in order to discuss clandestine operations said an explosive device had been smuggled into the Natanz plant, was detonated remotely, and took out both the primary and backup electrical systems.

The head of the Iranian Parliament’s energy committee, Fereydoun Abbasi, appeared to confirm that account in an interview with state television on Monday.

“The enemy’s plot was very beautiful,’’ he said. “I’m looking at it from a scientific point of view. They thought about this and used their experts and planned the explosion so both the central power and the emergency power cable would be damaged.”

A spokesman for the Atomic Energy Organization of Iran, Behrouz Kamalvandi, said Monday that the blast had exposed a hole so big that he fell into it, injuring his head, back, leg and arm.

Just how much damage was done is unclear.

Intelligence officials suggested that it would require many months for Iran to undo the damage.

Ali Akbar Salehi, the head of Iran’s Atomic Energy Organization, said emergency power had been restored at Natanz on Monday and enrichment had not stopped at the facility. But it may be running at a fraction of the level is was before.

“A large portion of the enemy’s sabotage can be restored, and this train cannot be stopped,” he told the Iranian news media on Monday.

But the attack, the latest in a series of major security breaches in the past year, has led to finger-pointing in Tehran and accusations of infiltration in the highest ranks of Iran’s security apparatus. The intelligence unit of the Islamic Revolutionary Guards Corps is responsible for both securing nuclear sites and protecting nuclear scientists.

Mr. Moghtdaie said his committee would investigate what he called “very obvious security infiltrations.”

Vice President Eshaq Jahangiri said the entity responsible for safeguarding Natanz against attacks must be held accountable. The attacks, he said, could produce “catastrophic consequences” for Iran’s reputation, economy and security.

And on social media, conservative commentators called for an overhaul in the leadership of the Atomic Agency and for the Supreme National Security Council to take over the nuclear file from the Foreign Ministry.

“The Natanz incident is either treason with infiltration or without,” tweeted one commentator, Seyed Peyman Taheri. “The cracks of your incompetence are being filled. Fix the holes.”

Some American officials, declining to speak on the record, expressed concern on Monday that the attack would drive the nuclear program more deeply underground, where it would be hard to reach. Iran already headed in that direction years ago, when it built a small plant deep inside a mountain near the city of Qum.

It was not the first time Iran’s adversaries had looked at interrupting power supplies to undercut the country’s nuclear or military programs. A similar technique was studied more than a decade ago by American officials but abandoned in favor of inserting malware, known as Stuxnet, as a more precise way to knock the centrifuges out of balance and destroy them.

The United States also had a complete battle plan for dealing with Iran that included attacks on its power grids. It was never implemented, but elements of the plan live on today.

More immediately, the leaking of details about Israeli involvement raised fears that Iran would seek to save face by mounting a stronger military response than usual.

“Once Israeli officials are quoted, it requires the Iranians to take revenge,” Danny Yatom, a former head of the Mossad, said in an interview Monday with a radio station run by the Israeli Army.

“There are actions that must remain in the dark,” he said.

Stephen Slick, who once managed the American intelligence agencies’ dealings with the Mossad, noted on Monday that the Israelis would have had many motives to strike.

“The varied policy messages likely include a pointed reminder to the U.S. and E.U. not to overlook Israel’s interests and freedom of action,” said Mr. Slick, now the director of the intelligence studies project at the University of Texas at Austin. “Iran was reminded of its persistent vulnerability to Israeli military and intelligence actions.”

In Israel, some also questioned whether the attack had served a domestic purpose for Mr. Netanyahu, rather than just a foreign policy objective.

Mr. Netanyahu is standing trial for corruption and is struggling to form a new coalition government after a general election last month that gave no party an overall majority. Some analysts said they believed that a very public confrontation with Iran might help Mr. Netanyahu persuade wavering coalition partners that now is not the time to remove an experienced prime minister.

“He may want to both build up his image and create a little bit of a foreign policy crisis, which then helps him solve the coalition crisis,” Chuck Freilich, a former deputy national security adviser in Israel, said on Monday.

But publicly, the United States and Israel kept up the image of friendly allies.

At a joint press briefing in Jerusalem, Mr. Austin did not mention Iran at all, while Mr. Netanyahu referred only obliquely to the attack on Sunday.

At a separate briefing with Defense Minister Benny Gantz, when an Israeli reporter asked Mr. Austin about the explosion, Mr. Gantz cut him off.

Iran has long maintained that its nuclear program is peaceful and aimed at energy development. But Israel sees it as an existential threat, since Iranian leaders have often called for Israel’s destruction.

“We both agree that Iran must never possess nuclear weapons,” Mr. Netanyahu said on Monday. “My policy as prime minister of Israel is clear. I will never allow Iran to obtain the nuclear capability to carry out its genocidal goal of eliminating Israel, and Israel will continue to defend itself against Iran’s aggression, and terrorism.”

In Washington, Ms. Psaki said she expected that the talks with Iran would resume Wednesday as planned.

“We expect them to be difficult and long,” she said. “We have not been given any indication about a change in participation for these discussions.”

Patrick Kingsley reported from Jerusalem, David E. Sanger from Washington and Farnaz Fassihi from New York. Reporting was contributed by Myra Noveck from Jerusalem, Steven Erlanger from Brussels and Eric Schmitt from Washington.