At a news conference in Istanbul, Mr. Araghchi reiterated that Iran “reserves all options to defend its security interests and people.” But he declined to be more specific — including about whether Iran would retaliate against U.S. military bases in the Middle East or move to close the Strait of Hormuz, a key global shipping lane beside southern Iran.

“I am not in a position to reveal what we are going to do in response,” he told reporters in Istanbul, saying that Iran was still assessing the damage from the strikes.

“There are a variety of options available to us,” he added. “And that’s it.”

When asked if the door to diplomacy was still open, he said that “this is not the case right now.”

“My country has been under attack, under aggression, and we have to respond based on our legitimate right of self-defense,” he said.

Mr. Araghchi said he would soon head to Moscow and planned to meet with President Vladimir V. Putin of Russia on Monday.

“Russia is a friend of Iran, and we enjoy strategic partnership and we always consult with each other and coordinate our positions,” he said.

The Kremlin has not yet commented on the U.S. strikes against Iran’s nuclear facilities.

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While Russia was quick to condemn Israel’s attacks on Iran, it has thus far stopped short of offering any concrete support. For Moscow, entangling itself could risk alienating the United Arab Emirates and Saudi Arabia, two increasingly important Russian partners, which would not welcome a more powerful Iran. And Russia’s forces are already tied down in Ukraine.

President Trump said the U.S. strikes on Fordo and two other Iranian nuclear sites, Natanz and Isfahan, had been successful, and that the facilities had been “completely and totally obliterated.” Iran confirmed that the sites had been hit, calling the attack “a violent act against international laws,” but did not comment on the extent of any damage.

The U.N.’s nuclear watchdog said it had not detected any increase in off-site radiation levels at the three Iranian nuclear sites following the U.S. attack, and that it was continuing to assess the situation and would provide further updates.

The Fordo site contains thousands of Iran’s most advanced centrifuges and is considered crucial for Iran to enrich uranium to 60 percent, a level from which enrichment to bomb grade — 90 percent or higher — can be relatively rapid. The government in Tehran kept the facility secret for years, but its existence emerged in Western intelligence and Iran formally disclosed the site in 2009.

In March 2023, the U.N. nuclear watchdog, the International Atomic Energy Agency, reported that it had discovered uranium that had been enriched to 83.7 percent purity at Fordo.

Iran has long maintained that its nuclear program, which is spread over several sites and facilities, is solely for peaceful use.

For years, however, Israel has viewed an assault on Fordo as central to its goal of destroying Iran’s ability to acquire nuclear weapons, something that Mr. Netanyahu has called an existential threat to his country.

Just before the Israeli government began its military campaign on June 13, the I.A.E.A. said that Iran was not complying with its nuclear nonproliferation obligations, the first time the agency had passed a resolution against the country in 20 years.

Yan Zhuang contributed reporting.

Israel and the United States are bracing for retaliation in the wake of the American attack on Iran’s nuclear program. There are fears that Iranian forces or their militant proxies across the Middle East would respond to the bombardment by attacking American troops or U.S. allies, which could lead to further fighting.

Iran’s Foreign Ministry said on Sunday morning that the country would defend its territory and security “by all force and means” against the U.S. attack, which it called “a grave and unprecedented violation” of international law.

As of Saturday evening, Israeli strikes had killed more than 400 people in Iran, according to the Iranian Health Ministry. Iran’s strikes on Israel killed at least 24 people, according to the Israeli government.

Before Sunday morning’s barrage, air-raid sirens blared out across northern and central Israel, warning of an impending Iranian missile attack, ordering much of the country to enter fortified bomb shelters.

After the attack subsided, search and rescue forces headed to sites across the country where Iranian missiles were believed to have evaded Israel’s air defenses, according to the Israeli military.

The Israeli police said officers were responding to impact sites in and around the coastal cities of Tel Aviv and Haifa. Magen David Adom published images showing paramedics amid buildings and cars damaged in the attack.

In a statement reported by Iranian state news media, the Islamic Revolutionary Guards Corps said that its forces had fired ballistic missiles at Israeli targets, including the country’s Ben Gurion International Airport, near Tel Aviv. The statement did not refer to the American attack on the Iranian nuclear sites.

In Ness Ziona — an affluent community in the Tel Aviv metropolitan area — an Iranian missile struck a quiet neighborhood, hollowing out some houses to bare cement shells. Cars smashed by the blast lay on the side of the road.

Sappers pushed residents and onlookers back from the area, stepping through broken glass and pieces of shrapnel. A firefighter guided a man who was carrying two crying children away from a red car with shattered windows.

Hadas Bar, 58, sat outside her damaged home, her hand shaking as she tried to steady a plastic cup of soda on a tilting patio table. She said she had taken shelter in a fortified safe room inside her house, along with her family and dog, when the air-raid warnings rang out.

Then came the blast, which reverberated throughout the house.

“The entire front of the house was torn off in the explosion,” Ms. Bar said.

So far, there was no evidence of Iranian retaliation against American targets in the immediate hours after the strike that Mr. Trump ordered. But Abbas Araghchi, Iran’s foreign minister, who just days ago was meeting with European leaders about the possibility of a negotiated end to the fighting, said in a social media post on Sunday morning that the U.S. attacks “will have everlasting consequences” and that “Iran reserves all options to defend its sovereignty, interest, and people.”

During the talks with the Europeans on Friday, Mr. Araghchi had said that “Iran is ready to consider diplomacy once again.” But it was unclear whether Tehran was serious about negotiating an end to the fighting that would be acceptable to the United States and Israel.

Mr. Araghchi said Iran would only return to talks “once the aggression is stopped and the aggressor is held accountable for the crimes committed.”

For days, Mr. Trump had publicly appeared to be holding back an American strike against Iran, while urging a “complete surrender” by Tehran’s leaders. Some of the president’s political allies cited the president’s longstanding criticism of “forever wars” as evidence that Mr. Trump wanted to embrace diplomacy and negotiation instead of a new military conflict.

And yet in announcing the strikes late on Saturday night, Mr. Trump rejected those urging him not to inject the United States directly into a war that could spiral throughout the region. American military officials said the B-2 bombers that dropped “bunker buster” bombs on the Iranian nuclear facilities had to travel about 37 hours to their targets.

Jonathan Panikoff, the director of the Atlantic Council’s Scowcroft Middle East Security Initiative, said that Mr. Trump’s attacks were unlikely to convince Iran’s leaders to embrace negotiation in the near future.

“The president’s speech this evening is more likely to be viewed by the Iranian regime as another threat rather than as an opening for diplomacy,” Mr. Panikoff said in a statement. “Iran is unlikely to believe it can simply capitulate to the United States — given that hard-liners in the Iranian regime might view such a decision as inappropriate.”

He added that “without something to be able to claim a reason for a lesser response, hard-liners in the Iranian regime may ultimately win the day, which could lead to a much more dangerous outcome.”

For his part, Mr. Trump appears to have concluded that Iran was not serious about a negotiated resolution after more than two months of failed diplomatic talks aimed at convincing the country’s leaders to give up their nuclear ambitions.

“Iran, the bully of the Middle East, must now make peace,” Mr. Trump said in remarks to the American people late Saturday. “If they do not, future attacks will be far greater and a lot easier. There will be either peace or there will be tragedy for Iran, far greater than we have witnessed over the last eight days.”

Here’s what some governments and leaders have said or done so far:

• United Nations: António Guterres, the U.N. secretary general, said on social media that he was “gravely alarmed,” describing the strikes as a “dangerous escalation in a region already on the edge — and a direct threat to international peace and security.” He warned of “a growing risk that this conflict could rapidly get out of control — with catastrophic consequences for civilians, the region, and the world.”

• European Union: The bloc’s top diplomat, Kaja Kallas, urged all sides to return to the negotiating table. “Iran must not be allowed to develop a nuclear weapon,” she said in a statement, adding that the E.U.’s foreign ministers would discuss the situation on Monday.

• Britain: “Iran can never be allowed to develop a nuclear weapon and the US has taken action to alleviate that threat,” Prime Minister Keir Starmer said on social media. He called on Iran to “return to the negotiating table and reach a diplomatic solution to end this crisis.”

• France: Jean-Noël Barrot, the foreign minister, expressed “concern” over the U.S. bombing and said that France was “not involved in these strikes.” France opposes a nuclear-armed Iran, and only diplomacy can bring a lasting solution, he said in a statement, adding that his country was urging restraint to avoid escalation.

• Spain: The government expressed “utmost concern” about the situation Middle East and called for a return to negotiations. Spain reiterated its call for restraint and respect for international law, the Spanish government statement.

• Australia: The government called for de-escalation and diplomacy, while echoing some of President Trump’s language about the danger of Iran’s nuclear program. “We have been clear that Iran’s nuclear and ballistic missile program has been a threat to international peace and security,” it said in a statement provided to the Australian Broadcasting Corporation, the country’s public broadcaster.

• Saudi Arabia: The foreign ministry called for de-escalation and restraint, saying that it was following the situation with “deep concern.” The statement stopped short of condemning the United States, although the foreign ministry had earlier condemned Israel’s attacks in Iran. Saudi Arabia depends heavily on American military support and hosts a U.S. military base that could be a target of Iranian retaliation.

• Lebanon: President Joseph Aoun of Lebanon urged restraint, adding that his country must stay out of the conflict.

• New Zealand: Foreign Minister Winston Peters urged diplomacy, saying in a statement: “Ongoing military action in the Middle East is extremely worrying, and it is critical further escalation is avoided.”

• South Korea: The country’s top security officials huddled on Sunday to discuss the potential impact on South Korea’s security and economy, a spokeswoman for President Lee Jae Myung said. Mr. Lee’s office also said that the president would not attend a NATO summit meeting in the Netherlands this week, citing pressing domestic issues and “uncertainty” spawned by rising tensions in the Middle East.

• Mexico: The Foreign Ministry called for “diplomatic dialogue and peace” among the parties involved in the conflict. “We reiterate our call to de-escalate tensions in the region,” it added.

• Cuba: President Miguel Díaz-Canel said on social media that the strikes were a “dangerous escalation” of the conflict in the Middle East and threatened to draw the world into crisis.

• Chile: President Gabriel Boric said the attack was illegal under international law. “We demand and need peace,” he said on social media.

• Venezuela: Foreign Minister Yván Gil described the attack as an “illegal, unjustifiable and extremely dangerous act of aggression” in a post on the social media platform Telegram.