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. 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 after the group considered a terrorist organization by many Western countries led an 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.

WATCH l U.S. Secretary of State Blinken reacts to Haniyeh killing:

Blinken says Israel-Hamas ceasefire 'imperative'

Secretary of State Antony Blinken, speaking in Singapore after reports of the assassination of Hamas political leader Ismail Haniyeh, says the U.S. will continue to work toward a ceasefire in Gaza.

The dramatic assassination of Hamas's top political leader threatened to reverberate throughout the region's intertwined conflicts. Most explosively, the strike in Tehran could push Iran and Israel into direct conflict 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."

There was no immediate reaction from the White House to the killing of Haniyeh.

Asked by reporters in Manila about the Tehran strike, U.S. Secretary of Defence Lloyd Austin said he had no "additional information to provide."

Israel often doesn't comment on assassinations carried out by its Mossad intelligence agency or strikes on other countries.

Haniyeh family members killed in April

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.

In April, an Israeli airstrike in Gaza killed three of Haniyeh's sons and four of his grandchildren.

Iranian media showed videos of Haniyeh and Iranian President Masoud Pezeshkian hugging after Pezeshkian's inauguration ceremony Tuesday.

Pezeshkian vowed his country would "defend its territory" and make the attackers "regret their cowardly action."

In the West Bank, the internationally backed Palestinian President Mahmoud Abbas condemned Haniyeh's killing, calling it a "cowardly act and dangerous development."

Hamas's 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." It said Israel "made a miscalculation by expanding the circle of aggression."

Speaking to the Associated Press, a Hamas spokesperson Sami Abu Zuhri said the loss of Haniyeh won't impact the group, saying it had emerged stronger after past crises and assassinations of its leaders.

Haniyeh left the Gaza Strip in 2019 and had lived in exile in Qatar. The top Hamas leader in Gaza is Yehya Sinwar.

Hamas is considered a terrorist organization by several Western countries, including Canada. Hamas led the Oct. 7 attack on southern Israel in which the Palestinian militant group killed 1,200 people, including several Canadians, and took some 250 others hostage, according to Israeli government tallies.

Over 100 hostages were repatriated in late 2023, but about 110 hostages remain unaccounted for, with the Israeli government believing about one-third are no longer alive.

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.

UN concerned about attack in Beirut

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

It could also enflame 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. 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.

WATCH l Retired U.S. Col. Mark Cancian on the latest Israel-Hezbollah hostilities:

Israel-Hezbollah escalation ‘unlikely,’ says retired U.S. colonel | Canada Tonight

The Israeli military said late on Tuesday that Israeli air force fighter jets have killed Hezbollah's most senior military commander and head of its strategic unit, Fuad Shukr. Mark Cancian, a retired U.S. colonel and a senior adviser with the Center for Strategic and International Studies, a U.S. think-tank, says an escalation is ‘unlikely, but not impossible.’

UN Secretary General General Antonio Guterres's spokesperson Stephane Dujarric expressed "grave concern" over the strikes in the densely populated neighbourhood.

"As we await further clarity on the circumstances, we again urge the parties to exercise maximum restraint and call on all concerned to avoid any further escalation," said Dujarric.

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.