Avenging Hamas leader Ismail Haniyeh's assassination is "Tehran's duty" because it occurred in the Iranian capital, Iran's Supreme Leader Ali Khamenei said on Wednesday.
He said Israel had provided the grounds for "harsh punishment" for itself.
Iran’s President Masoud Pezeshkian condemned the killing.
He said in a post on the X platform that his country will defend its territorial integrity and make those responsible regret their actions.
Hamas has blamed Israel for the assassination.
Hamas said Haniyeh was killed at his residence in Tehran in an Israeli airstrike after he attended the swearing-in ceremony of Iran’s new president. Israel has not commented on the accusation.
Turkish President Recep Tayyip Erdogan condemned the assassination, saying the killing would not break Palestinians' will.
"This assassination is a vileness that aims to disrupt the Palestinian cause, Gaza's noble resistance and our Palestinian siblings' rightful struggle, to break the will of Palestinians, and to intimidate them," Erdogan said on social media platform X. "However, just as until today, the Zionist barbarism will not reach its goals."
Turkish Foreign Minister Hakan Fidan said he was "deeply saddened" to hear that Haniyeh had been "martyred", adding that he had become a symbol of Palestinian resistance.
"He had devoted his life to the Palestinian cause, and to bringing peace and tranquility to Palestine," Fidan added on social media platform X, sharing a photograph of himself and Haniyeh.
Russia's foreign ministry also condemned the killing and called for restraint to stop the Middle East tipping into a large-scale war.
China condemned the assassination, with its foreign ministry warning the incident could lead to further regional instability.
