Hamas leader Ismail Haniyeh was assassinated in Iran on Wednesday, and several foreign governments criticized the attack as a "heinous crime" that will "lead to further escalation of tensions" in the region.

Nobody immediately claimed responsibility for the attack, but Israel was quickly blamed after pledging to kill Haniyeh and other Hamas leaders over the terrorist group's Oct. 7 attack on the Jewish State.

Israel did not immediately comment, but it usually does not make public comments on assassinations carried out by their Mossad intelligence agency.

Iranian Supreme Leader Ali Khamenei said it is "Iran's duty to avenge Haniyeh's blood because he was martyred on our soil."

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"We will make the terrorist occupiers regret the cowardly assassination, we will defend our honor," Iranian President Masoud Pezeshkian said.

Iran Foreign Ministry spokesman Nasser Kanaani said: "Haniyeh's martyrdom in Tehran will strengthen the deep and unbreakable bond between Tehran, Palestine, and the resistance."

The Hamas leader's son, Abdul Salam Haniyeh, said his father "survived four assassination attempts during his patriotic journey, and today Allah has granted him the martyrdom that he always wished for."

"He was very keen to establish national unity and strived for the unity of all Palestinian factions, and we affirm that this assassination will not deter the resistance, which will fight until freedom is achieved," he added.

Senior Hamas official Sami Abu Zuhri said: "This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas and the will of our people and achieve fake goals. We confirm that this escalation will fail to achieve its objectives."

"The assassination of Haniyeh will be met with a harsh and painful response," Iran's elite Revolutionary Guards said.

Deputy Russian foreign Minister Mikhail Bogdanov called the attack "an absolutely unacceptable political murder, and it will lead to further escalation of tensions."

"Qatar strongly condemns the assassination of Dr. Ismail Haniyeh in Tehran and considers this a heinous crime and a dangerous escalation, as well as a major violation of international and humanitarian laws," the Qatar Foreign Ministry said in a statement. 

"The foreign ministry affirms that this assassination and Israel's reckless, continuous targeting of civilians in Gaza will push the region into a circle of chaos, as well as limit the prospects of peace."

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The Turkish Foreign Ministry said: "Hamas is a concept and an institution and not persons. Hamas will continue on this path regardless of the sacrifices and we are confident of victory."

Turkey's Ministry of Foreign Affairs added, in part, on the social media platform X that Haniyeh "was a figure who became the symbol of the glorious Palestinian resistance. His cherished memory will live on in the just cause of the Palestinian people."

U.S. Defense Secretary Lloyd Austin said the U.S. government would seek to ease tensions but that it would help defend Israel if it were attacked.

Israeli Prime Minister Benjamin Netanyahu will hold a situation assessment hearing with the heads of the defense establishment on Wednesday in the wake of the assassination. Israel will also increase security at Jewish institutions around the world.

Reuters contributed to this report.