(WASHINGTON) — An attack on one of Iran’s most sensitive nuclear facilities has threatened to unravel President Joe Biden’s diplomatic efforts to revive the Iran nuclear deal — and to inflame tensions in a region already on edge.

The sabotage on Iran’s underground site Natanz was first reported by Iranian state media Sunday, with its nuclear chief calling it an act of “nuclear terrorism.” While the full details are unknown, Natanz was reported to have experienced a blackout, allegedly the result of a cyber attack on its electrical grid.

The White House said Monday that the U.S. was not involved in the incident at all. Suspicions have instead fallen on Israel, especially after several Israeli media outlets first reported Sunday that an Israeli attack and ensuing explosion took entire sections of the nuclear site offline.

The blackout comes just days after Iran and the U.S. began indirect negotiations over both countries re-entering the 2015 nuclear deal, possibly imperiling those talks if Iran sees growing pressure on its nuclear program as a reason to push it further.

But both the U.S. and Iran signaled full-steam ahead on Monday with their indirect negotiations, while Iran’s foreign minister accused Israeli Prime Minister Benjamin Netanyahu’s government of trying to derail those talks.

“This act of sabotage damaged not only Natanz, but also the Biden administration’s plan to return to compliance with the nuclear deal,” said Kelsey Davenport, director for nonproliferation policy at the Arms Control Association, a Washington think tank. “The United States and Iran mustn’t let this attack derail the progress being made in Vienna.”

Those talks in Austria’s capital are set to resume Wednesday, with the U.S. meeting European parties still in the nuclear deal, while Iran meets all the remaining parties, including Russia and China.

Former President Donald Trump withdrew from the deal in 2018, re-imposing strict sanctions and pushing to destroy the agreement. One year later, Iran began taking steps in violation of its commitments — enriching more uranium at higher levels using more centrifuges and more advanced ones than what is allowed.

While Biden has called for both sides returning to the deal — and then pursuing a “longer and stronger” follow-on agreement — talks only started last week through intermediaries, as Iran refuses to meet the U.S. face-to-face so far. While officials described some progress, there remains a large gap over who should take the first step. Iran insists that the U.S. must lift all Trump-era sanctions first, but the U.S. has said there will not be sanctions relief until Iran restores the limits on its nuclear program — proposing a mutual return instead.

Natanz has been at the heart of those restrictions.

Biden’s Defense Secretary Lloyd Austin was in Israel on Sunday, prompting speculation that Israel gave the U.S. advanced notice or was somehow involved. But White House press secretary Jen Psaki said Monday the U.S. was “not involved in any manner.”

So far, there has been no change in plans for Wednesday’s meetings.

“Our focus is of course on the diplomatic path forward. We have not been given any indication that attendance at the discussions… has changed,” Psaki said.

Iran’s Foreign Minister Javad Zarif vowed “revenge against the Zionists,” a reference to Israel, but said the incident would not affect talks over the nuclear deal.

“The Zionists want to take revenge because of our progress in the way to lift sanctions,” he said, according to state television. “We will not allow this act of sabotage to affect the nuclear talks.”

But if talks continue, there could be an escalation in the region. Netanyahu warned last week that agreements with Iran “are worthless” and that Israel will do whatever is necessary “to prevent those who wish to destroy us from carrying out their plans.”

There have been a series of attacks on Iran’s nuclear program in the last year that have been traced back to Israel — a signal that the country is willing to escalate and act boldly to dismantle Iran’s nuclear program. Last July, there was a mysterious explosion at its advanced centrifuge production plant, while in November, Iran accused Israel of assassinating a nuclear scientist who launched its military nuclear program years ago.

The two countries have also been engaged in a shadow war at sea. Last Tuesday, an Iranian cargo ship was hit by an explosion, which Tehran blamed Israel for as well. The ship was said to be a floating base for Iranian forces engaged in Yemen’s war.

With the director of Mossad, Israel’s spy service, at his side on Sunday, Netanyahu urged Israeli forces to “continue in this direction” and praised their strength against foreign threats.

While that may put him at odds with Biden and his push for nuclear diplomacy, Austin and Netanyahu tried to project close ties, with Austin offering his “personal pledge to strengthening Israel’s security.”

But Netanyahu repeatedly warned of the threat from the “fanatical regime in Iran” and said Israel will “continue to defend itself against Iran’s aggression and terrorism” — while Austin made no mention of Iran in his brief remarks.

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