TEHERAN - Iran's Supreme Leader Ali Khamenei on Wednesday vowed to avenge Hamas Politburo Chief Ismail Haniyeh's death in the Iranian territory.

He made the remarks in a message published on his website to extend condolences to the Palestinian people and resistance front over the "martyrdom" of Haniyeh and one of his companions in an alleged Israeli attack earlier in the day in Tehran.

He stressed that Israel prepared the ground for a "harsh punishment" for itself.

In two statements on Wednesday, Iran's Islamic Revolution Guards Corps (IRGC) accused Israel of assassinating Haniyeh and one of his bodyguards at their residence in Teheran in a "terrorist attack."

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It extended condolences over the "martyrdom" of Haniyeh and his bodyguard, adding Haniyeh was among the dignitaries invited to Tehran to attend the swearing-in ceremony of Iranian President Masoud Pezeshkian on Tuesday and had, over the past years, paid numerous visits to Iran.

The IRGC condemned the "criminal act" as in defiance of international regulations and laws, saying Israel "will receive a harsh and painful response" from Iran.

The Iranian president also, in a post on social media platform X, expressed condolences over Haniyeh's "martyrdom," saying Iran would defend its territorial integrity, dignity, honor, and pride and make the Israelis, which he described as "terrorist occupiers," regret their "cowardly move."

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The Iranian government has declared a three-day national mourning for the Hamas leader, according to the official news agency IRNA.

Haniyeh's funeral procession will be held in Teheran on Thursday morning, the IRNA said.