Hamas leader Ismail Haniyeh has been killed by a pre-dawn air strike in the Iranian capital, Iran and the militant group say, blaming Israel for a shock assassination that risks escalating the conflict even as the US and other nations were scrambling to prevent an all-out regional war.
There was no immediate comment from Israel, which has vowed to kill Haniyeh and other Hamas leaders over the group's October 7 attack on southern Israel.
The strike on Wednesday came just after Haniyeh had attended the inauguration of Iran's new president in Tehran, Iran said.
The dramatic killing of Hamas's top political leader threatened to reverberate on multiple fronts.
The blow of striking Haniyeh in Tehran could trigger direct retaliation against Israel by Iran.
President Masoud Pezeshkian vowed to make Ismail Haniyeh's killers "regret their cowardly action". (AP PHOTO)
Iran's leadership vowed retaliation, with Supreme Leader Ali Khamanei saying avenging the strike was "Tehran's duty" and President Masoud Pezeshkian promising to make the attackers "regret their cowardly action".
The bitter regional rivals had an unprecedented exchange of strikes on each other's soil in April after Israel hit Iran's embassy in Damascus, but international efforts succeeded in containing that cycle of retaliation before it spun out of control.
Hamas could pull out of negotiations for a ceasefire and hostage release deal in the 10-month-old war in Gaza, which US mediators had said were making progress.
And the killing could inflame already heightening tensions between Israel and Iran's powerful Lebanese ally Hezbollah, which international diplomats were trying to contain after a weekend rocket attack that killed 12 young people in the Israeli-controlled Golan Heights.
Hours before the Tehran strike, Israel carried out a rare strike in the Lebanese capital on Tuesday that it said killed a top Hezbollah commander allegedly behind the rocket strike.
Hezbollah, which denied any role in the Golan strike, said it was still searching for the body of Fouad Shukur in the rubble of the building that was hit in a Beirut suburb that is the group's stronghold.
The White House had no immediate reaction to the killing of Haniyeh.
Asked by reporters in Manila about the Tehran strike, US Secretary of Defense Lloyd Austin said he had no "additional information to provide", but he expressed hope for a diplomatic solution on the Israeli-Lebanon border.
Ismail Haniyeh was in Tehran for the Iranian president's swearing-in ceremony when he was killed. (AP PHOTO)
"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."
An Israeli military spokesman declined to comment.
Israel has repeatedly vowed to eliminate Hamas leaders wherever they are for the October 7 attack in which Hamas-led militants killed 1200 people and took some 250 others hostage.
Iranian media showed videos of Haniyeh and the Iranian president hugging after Pezeshkian's inauguration ceremony on Tuesday.
Hours later, the strike hit a residence Haniyeh uses in Tehran, killing him, Hamas said.
Hamas spokesman Sami Abu Zuhri said Haniyeh's killing would not affect the group, adding Israel was "spreading chaos and evil" in the region.
"The occupation will not succeed in achieving its goals," he told the Associated Press.
Along with October 7 mastermind Yehya Sinwar, left, Ismail Haniyeh was among Hamas' top leaders. (AP PHOTO)
Haniyeh left the Gaza Strip in 2019 and had lived in exile in Qatar.
The top Hamas leader in Gaza is Yehya Sinwar, who masterminded the October 7 attack.
Taher al-Nounou, Haniyeh's media adviser, told Al-Jazeera TV: "This is a turning point for the conflict."
He said Israel and "those who stand with it - and by this we mean the United States" would bear responsibility.
In the West Bank, the internationally backed Palestinian President Mahmoud Abbas condemned Haniyeh's killing, calling it a "cowardly act and dangerous development".
In April, an Israeli air strike in Gaza killed three of Haniyeh's sons and four of his grandchildren.
In an interview with the Al Jazeera satellite channel at the time, Haniyeh said the killings would not pressure Hamas into softening its positions amid ongoing ceasefire negotiations with Israel.
In Israel's war against Hamas since the October attack, more than 39,360 Palestinians have been killed and more than 90,900 wounded, according to the Gaza health ministry, whose count does not differentiate between civilians and combatants.