"The Islamic Republic of Iran will defend its territorial integrity, honor, pride, and dignity, and make the terrorist invaders regret their cowardly action," said Pezeshkian in a post on X, where he mourned Haniyeh as "a brave leader."

Iran's supreme leader Ayatollah Ali Khamenei vowed "harsh punishment" for Israel after the killing of Haniyeh in Tehran.

"With this action, the criminal and terrorist Zionist regime prepared the ground for harsh punishment for itself, and we consider it our duty to seek revenge for his blood as he was martyred in the territory of the Islamic Republic of Iran," he said in a statement carried by official news agency IRNA.

Iranian Foreign Ministry Spokesman Nasser Kanaani said Haniyeh's blood will not go in vain.

Haniyeh was a tireless fighter who combated the Israeli regime throughout his life to free the holy Quds, Kanaani stated on Wednesday.

Kanaani expressed condolences over Haniyeh’s death to the great Palestinian nation, Hamas, all resistance groups, and the countries that support the Palestinian cause.

The Islamic Revolution Guards Corps said the attack is under investigation and the results will be announced later.

Haniyeh, who was in Tehran to attend the new Iranian president’s swearing-in ceremony, was killed in a special residence in northern Tehran after being hit by an aerial projectile in the early hours of Wednesday, according to Iranian media.

The funeral service of the Hamas chief will be held in the Iranian capital probably on Thursday 1 August, Iranian news agency Tasnim reported.