Hamas leader Ismail Haniyeh was killed by a predawn airstrike in the Iranian capital Wednesday, Iran and the militant group said, blaming Israel for a shock assassination that risks escalating the conflict even as the U.S. and other nations were scrambling to prevent an all-out regional war.

Iran's supreme leader vowed revenge against Israel. There was no immediate comment from Israel, which has pledged to kill Haniyeh and other Hamas leaders over the group’s Oct. 7 attack on southern Israel.

The strike came just after Haniyeh had attended the inauguration of Iran’s new president in Tehran — and only hours after Israel targeted a top commander in Iran's ally Hezbollah in the Lebanese capital Beirut.

The assassination of Hamas’ top political leader was potentially explosive amid the region's volatile, intertwined conflicts — because of its target, its timing and the decision to carry it out in Tehran. Most dangerous was the potential to push Iran and Israel into direct confrontation if Iran retaliates.

“We consider his revenge as our duty,” Iranian supreme leader Ayatollah Ali Khamenei said in a statement on his official website. He said Israel had “prepared a harsh punishment for itself” by killing "a dear guest in our home."

Bitter regional rivals, Israel and Iran risked plunging into war earlier this year when Israel hit Iran’s embassy in Damascus in April. Iran retaliated and Israel countered in an unprecedented exchange of strikes on each other's soil, but international efforts succeeded in containing that cycle before it spun out of control.

Haniyeh's killing could also prompt Hamas to pull out of negotiations for a cease-fire and hostage release deal in the 10-month-old war in Gaza, which U.S. mediators had said were making progress.

And it could inflame already heightening tensions between Israel and Hezbollah — which international diplomats were trying to contain after a weekend rocket attack that killed 12 young people in the Israeli-controlled Golan Heights.

Tuesday evening, Israel carried out a rare strike in the Lebanese capital that it said killed a top Hezbollah commander allegedly behind the rocket strike. Hezbollah, which denied any role in the Golan strike, said Wednesday that it was still searching for the body of Fouad Shukur in the rubble of the building that was hit in a Beirut suburb, killing two women and two children, according to the Lebanese Health Ministry.

There was no immediate reaction from the White House to the killing of Haniyeh. A key question was whether Israel told its top ally the U.S. ahead of time about the strike.

Asked about Haniyeh's killing during a visit to Singapore, Secretary of State Antony Blinken said, "This is something we were not aware of or involved in.”

Blinken declined to speculate on the impact Haniyeh’s death would have on efforts to secure a cease-fire in Gaza between Israel and Hamas. But he said the incident made it more important to reach a deal to ease the suffering of civilians, free hostages held by Hamas and prevent the conflict from escalating.

“First, this is something we were not aware of or involved in,” Blinken said in an interview with Channel News Asia, according to a transcript provided by the State Department. “I’ve learned over many years never to speculate on the impact that an event has had on something else. So I can’t tell you what this means,” Blinken added.

Blinken also said he would not speculate about the impact on cease-fire efforts, adding: "But I can tell you that the imperative of getting a cease-fire, the importance that that has for everyone, remains.”

In Manila, Defense secretary Lloyd Austin said he still had hopes for a diplomatic solution on the Israeli-Lebanon border.

“I don’t think that war is inevitable,” he said. “I maintain that. I think there’s always room and opportunity for diplomacy, and I’d like to see parties pursue those opportunities.”

But international diplomats trying to defuse tensions were alarmed. One Western diplomat, whose country has worked to prevent an Israeli-Hezbollah escalation, said the double strikes in Beirut and Tehran have “almost killed” hopes for a Gaza cease-fire and could push the Middle East into a “devastating regional war.” The diplomat spoke on condition of anonymity to discuss the sensitive situation.

An Israeli military spokesman declined to comment. Israel often doesn’t comment on assassinations carried out by its Mossad intelligence agency or strikes on other countries.

In a statement by his office, Israeli Defense Minister Yoav Gallant said Israel doesn't want war after its strike on the Hezbollah commander in Beirut, “but we are preparing for all possibilities.” He did not mention the Haniyeh killing.

The killing of Haniyeh abroad comes as Israel has not had a clear success in killing the group’s top leadership in Gaza, who are believed to be primarily responsible for planning the Oct. 7 attack, after nearly 10 months of fighting in the enclave.

Earlier this month, Israel carried out a strike in Gaza targeting the head of Hamas’ military wing, Mohammed Deif, killing at least 90 Palestinians living in nearby tents, according to Gaza health authorities. Israel said it believed Deif was killed, but neither it nor Hamas has confirmed his death. More elusive has been Hamas’ top leader in Gaza, Yehya Sinwar, believed to be the mastermind of Hamas' brutal surprise assault into southern Israel, during which militants killed some 1,200 people and abducted 250 others.

Haniyeh left the Gaza Strip in 2019 and had lived in exile in Qatar. Israel has targeted Hamas figures in Lebanon and Syria during the war — but going after Haniyeh in Iran was vastly more sensitive. But Israel has operated there in the past: It is suspected of running a yearslong assassination campaign against Iranian nuclear scientists. In 2020, a top Iranian military nuclear scientist, Mohsen Fakhrizadeh, was killed by a remote-controlled machine gun while traveling in a car outside Tehran.

During Haniyeh's last hours in Iran — a close ally of Hamas — he was smiling and clapping at the inauguration ceremony of the new President Masoud Pezeshkian. AP photos showed him seated alongside leaders from the Palestinian Islamic Jihad militant group and Hezbollah, and Iranian media showed him and Pezeshkian hugging. Haniyeh had met earlier with Khamenei.

Hours later, the strike hit a residence Haniyeh uses in Tehran, killing him, Hamas said in a statement. One of his bodyguards was also killed, Iranian officials said.

Iran’s powerful Revolutionary Guard warned Israel will face a “harsh and painful response” from Iran and its allies around the region because of the killing. An influential Iranian parliamentary committee on national security and foreign policy was to hold an emergency meeting on the strike later Wednesday.

Hamas’ military wing said in a statement that Haniyeh’s assassination “takes the battle to new dimensions and will have major repercussions on the entire region.”

Israeli Prime Minister Benjamin Netanyahu has said Israel will continue its devastating campaign in Gaza until Hamas is completely eliminated. Israel's bombardment and offensives in Gaza have killed more than 39,360 Palestinians and wounded more than 90,900, according to the Gaza Health Ministry, whose count does not differentiate between civilians and combatants.

After months of pounding, Hamas has shown its fighters can still operate in Gaza and fire volleys of rockets into Israel. But it is unclear if it has the capacity to step up attacks in retaliation over Haniyeh's killing.

Instead, the impact may be regional. Besides a direct retaliation on Israel, Iran could work to hike up attacks through its allies, a coalition of Iranian-backed groups known as the “Axis of Resistance,” including Hezbollah, Hamas, mainly Shiite militias in Iraq and Syria and the Houthi rebels who control much of Yemen.

As a show of support for Hamas in the Gaza war, Hezbollah has been exchanging fire almost daily with Israel across the Israeli-Lebanese border in a simmering but deadly conflict that has repeatedly threatened to escalate into all-out war. The Houthis and Iraqi and Syrian militias have also fired rockets and drones at Israel and at American bases in the region, though most have been intercepted.

A strike Tuesday night southwest of the Iraqi capital Baghdad killed four members of one Iranian-backed militia, Kataib Hezbollah, which has targeted U.S. bases previously, according to Iraq's Popular Mobilization Forces, a militia coalition. It accused the U.S. of being behind the strike. U.S. officials did not immediately comment.