TEL AVIV — Defense Secretary Lloyd Austin, making the first visit to Israel by a senior Biden administration official, said Sunday that the U.S.-Israeli relationship was "enduring and ironclad," amid growing Israeli concern over American efforts to revive the 2015 Iranian nuclear deal.

The start of the two-day visit, in which Austin is scheduled to meet with several leading figures of Israel’s defense establishment to discuss the sale of U.S. arms to Israel, coincided with reports of an overnight power outage at Natanz, a highly sensitive Iranian nuclear site, just hours after it began operating new advanced centrifuges capable of enriching uranium more quickly.

“This is a relationship that is built on trust, which has developed over decades of cooperation,” Austin said in a joint statement with Israeli Defense Minister Benny Gantz at a military base in Tel Aviv.

Gantz said Israel “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.”

In Iran, Ali Akbar Salehi, head of the civilian Atomic Energy Organization of Iran, suggested the blackout at Natanz was the result of a deliberate attack. He called it “nuclear terrorism” and said Iran reserved “the right to respond to the perpetrators,” according to comments carried by Iran’s state television and published online late Sunday. Salehi did not say who was responsible for the attack.

Behrouz Kamalvandi, a spokesman for the nuclear agency, told state TV that the incident caused no injuries or environmental damage.

For years, Israel has been suspected in high-profile explosions, assassinations and other forms of sabotage intended to delay the expansion of Iran’s nuclear program. They included an attack on Natanz last year, which caused a massive explosion.

Recent weeks have seen increasing reports of maritime attacks.

An explosion last week damaged an Iranian ship in the Red Sea just hours before Iran and the United States launched indirect talks in Vienna. The discussions were aimed at recommitting Tehran to the terms of the 2015 accord and lifting the sanctions imposed by President Donald Trump.

Israel issued no comment on any of the incidents, as is common practice.

“The fight against Iran and its metastases and against Iranian armament is a huge task,” Prime Minister Benjamin Netanyahu said in a meeting with Israeli security chiefs. “The situation that exists today will not necessarily be the situation that will exist tomorrow.”

Israeli media reported that the Austin visit was intended to deliver a message from Washington to Israeli leaders: Do not hurt American attempts to reengage Iran.

“It’s not for nothing that the Biden administration chose the first visit of a senior official to Israel to be a security rather than a political figure,” Itay Blumenthal and Itamar Eichner wrote on the Israeli news site Yediot Ahronot. “The message is that the United States is interested in coordinating with Israel on security issues, but is less attentive to it now on political issues.”

Since Trump withdrew from the nuclear deal in 2018, Iran has insisted it is still committed to the agreement. But it has said it would progressively abandon some elements of the deal in retaliation for the U.S. pullout and the Trump administration’s imposition of hundreds of new sanctions.

High-resolution satellite photos taken last year showed Iran has been expanding its uranium enrichment complexes at Natanz, including the construction of new tunnels and roads, and at Fordo, another underground nuclear site.

The International Atomic Energy Agency reported in November that Iran had stockpiled more than 2.4 tons of uranium and enriched up to 4.5 percent, which is 12 times the weight limit set by the 2015 deal. Since January, it has produced 55 kilograms of 20 percent enriched uranium, Kamalvandi, the Iranian official, told state television Wednesday.

Lt. Gen. Aviv Kochavi, chief of staff of the Israeli military, said Sunday that the country’s military “operations in the Middle East are not hidden from the eyes of the enemy. They are watching us, seeing [our] abilities and weighing their steps with caution. We are on guard, with improved offensive abilities.”

Kareem Fahim in Istanbul contributed to this report.