Iran’s supreme leader Ayatollah Ali Khamenei has vowed revenge on Israel over the killing of Hamas’ political chief.

Khamenei said Israel had “prepared a harsh punishment for itself” after Ismail Haniyeh was killed in a pre-dawn air strike in the Iranian capital Tehran.

The Iranian leader said in a statement on his official website: “We consider his revenge as our duty,” adding Mr Haniyeh had been “a dear guest in our home”.

The shock assassination risks escalating the Israel-Hamas conflict even as the US and other nations scramble to prevent an all-out regional war.

Israeli Prime Minister Benjamin Netanyahu said Israel “will exact a very heavy price from any aggression against us on any front” but did not mention the killing.

“There are challenging days ahead,” he added.

The Iranian President and Mr Haniyeh shake hands
Iranian President Masoud Pezeshkian met Mr Haniyeh recently (Iranian Presidency Office via AP)

The strike came just after Mr Haniyeh had attended the inauguration of Iran’s new President in Tehran.

The dramatic killing threatens to reverberate on multiple fronts, with direct retaliation from Iran one possibility.

New Iranian President Masoud Pezeshkian vowed his country would “defend its territory” and make the attackers “regret their cowardly action”.

The two 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.

An influential Iranian parliamentary committee on national security and foreign policy is due to hold an emergency meeting on the strike later on Wednesday.

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 enflame 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.

Hamas leader in close-up
Turkey condemned Mr Haniyeh’s killing (AP)

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 Golan Heights rocket strike.

Hezbollah, which denied any role in the Golan strike, said it had found the body of Fouad Shukur in the rubble of the building that was hit in a Beirut suburb that is the group’s stronghold.

Hamas said earlier: “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.”

In another statement, the group quoted Mr 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”.

Palestinians inspect the rubble of a school
Tens of thousands of Palestinians have been killed in the wake of the October 7 assault in Israel (AP)

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

Asked about the attack during a visit to Singapore, US secretary of state Antony Blinken said: “This is something we were not aware of or involved in.”

Speaking to Channel News Asia, Mr Blinken said he would not speculate about the impact on ceasefire efforts.

“But I can tell you that the imperative of getting a ceasefire, the importance that that has for everyone, remains,” he added.

Asked by reporters in Manila about the Tehran strike, US secretary of defence Lloyd Austin said he had no “additional information to provide”. But he expressed hope 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.”

However, international diplomats trying to defuse tensions were alarmed. One Western official, 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 ceasefire and could push the Middle East into a “devastating regional war”.

In a statement by his office, Israeli defence minister Yoav Gallant said Israel does not 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.

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

Political factions in the occupied territory called for strikes in protest at the killing.

Turkey has strongly condemned the “heinous assassination” of the Hamas leader, attributing the killing to the Israeli government.

A foreign ministry statement said the killing had shown that Israeli Prime Minister Benjamin Netanyahu’s government “has no intention of achieving peace”.