TEHRAN — Iran's Supreme Leader Ayatollah Ali Khamenei has sworn revenge on Israel over the killing of Hamas' political chief, in a shock assassination that risks escalating the conflict.

Khamenei said Israel "prepared a harsh punishment for itself" after Ismail Haniyeh was killed in a predawn airstrike in the Iranian capital Tehran on Wednesday.

"We consider avenging him as our duty," Khamenei said in a statement on his official website, saying Haniyeh was "a dear guest in our home".

His comments came after Hamas put the blame for the attack firmly at the feet of Israel.

In a statement, the group said that Haniyeh was killed "in a Zionist airstrike on his residence in Tehran after he participated in the inauguration of Iran's new president".

"Hamas declares to the great Palestinian people and the people of the Arab and Islamic nations and all the free people of the world, brother leader Ismail Ismail Haniyeh a martyr," the statement said.

Israel had vowed to kill Haniyeh and other leaders of Hamas over the group's 7 October attack on Israel.

An Israeli military spokesman declined to comment; Israel often doesn't when it comes to assassinations carried out by their Mossad intelligence agency.

In another statement from Hamas, the group quoted Haniyeh as saying that the Palestinian cause has "costs" and "we are ready for these costs: martyrdom for the sake of Palestine, and for the sake of God Almighty, and for the sake of the dignity of this nation."

Hamas' military wing said 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 AP, a Hamas spokesman 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, who masterminded the 7 October attack.

In April, an Israeli airstrike 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 cease-fire negotiations with Israel.

Haniyeh was in Tehran to attend Iran's President Masoud Pezeshkian’s swearing-in ceremony on Tuesday, along with other Hamas officials and officials from Hezbollah and allied groups.

Iran gave no details on how Haniyeh was killed, and the Guard said the attack was under investigation.

The killing of Haniyeh comes after Israel carried out a rare strike on Beirut, which it said killed Fouad Shukur, a top Hezbollah military commander. Hezbollah has not confirmed Shukur's death in the strike, which also killed at least one woman and two children and wounded dozens of people.

The strike came amid escalating hostilities with the Lebanese militant group. The US also blames Shukur for planning and launching the deadly 1983 Marine bombing in the Lebanese capital.

Elsewhere, Turkey's Ministry of Foreign Affairs said it condemned the "vile assassination" of Haniyeh, which it said aims to spread the war in Gaza to a regional level.

"We offer our condolences to the Palestinian people who gave hundreds of thousands of martyrs like Haniyeh so that they could live in peace in their own homeland, under the roof of their own state," the ministry said in a statement shared on X.

"It has been revealed once again that the Netanyahu Government has no intention of achieving peace," it added.

There was no immediate reaction from the White House. The apparent assassination comes at a precarious time, as the Biden administration has tried to push Hamas and Israel to agree to at least a temporary cease-fire and hostage-release deal.

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. — Euronews