The international community was left stunned by the assassination of Hamas political leader Ismail Haniyeh in an Israeli airstrike in Tehran on Wednesday.
The killing comes at a critical time in Israel's genocidal war on Gaza and prompted many countries to warn that it could trigger a wider regional war.
Iran's supreme leader Ayatollah Ali Khamenei vowed "harsh punishment" for Israel over Haniyeh's killing in the capital 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," Khamenei said in a statement carried by the IRNA official news agency.
Newly elected Iranian President Masoud Pezeshkian said that Iran "will defend its territorial integrity, honor, pride and dignity, and make the terrorist invaders regret their cowardly action."
Both actors Qatar and Türkiye were also quick to condemn the killing and warned of "chaos."
Haniyeh was based in Qatar, which has been a mediator in the Gaza conflict but also spent time in Türkiye after going into exile. Qatar's Foreign Ministry called the killing a "heinous crime" and "shameful assassination."
The ministry said: "This assassination and the reckless Israeli behavior of continuously targeting civilians in Gaza will lead to the region slipping into chaos and undermine the chances of peace."
Türkiye's President Recep Tayyip Erdogan condemned what he called the "perfidious assassination" of his "brother" Haniyeh.
"This shameful act aims to sabotage the Palestinian cause, the glorious Gazan resistance and our Palestinian brothers' just fight, and to intimidate Palestinians," Erdoğan added in a social media post.
Egypt in its reaction said that the Israeli escalation indicated a lack of political will for de-escalation.
A statement from the Egyptian Foreign Ministry said this escalation, along with making no progress in Gaza cease-fire talks was complicating the situation.
"The coincidence of this regional escalation with the lack of progress in the ceasefire negotiations in Gaza increases the complexity of the situation and indicates the absence of Israeli political will to calm it down," the statement said.
"It undercuts the strenuous efforts made by Egypt and its partners to stop the war in the Gaza Strip and put an end to the human suffering of the Palestinian people," it added.
Meanwhile, U.S. Secretary of State Antony Blinken said that reaching a cease-fire in the Gaza war "is the enduring imperative" after the killing of Haniyeh.
Germany stressed international calls for restraint to avoid "a regional conflagration," with a foreign ministry spokesman saying: "The logic of tit-for-tat reprisals is the wrong path."
Russia, China condemn
Haniyeh went to Moscow in September 2022 for talks on the Israel-Palestinian conflict while Hamas and rival Palestinian faction Fatah held talks in Beijing last week.
"It is a completely unacceptable political assassination, and this will lead to a further escalation of tensions," Russia's Deputy Foreign Minister Mikhail Bogdanov told the state-run RIA Novosti news agency.
Konstantin Kosachev, the vice-president of Russia's upper house Federation Council, predicted a "sudden escalation of mutual hatred" in the Middle East.
China's Foreign Ministry spokesman Lin Jian said: "We are highly concerned about the incident and firmly oppose and condemn the assassination."
Major shock
The killing reverberated across the Middle East. Egypt's foreign ministry said that Israel's "dangerous escalation" in recent days "risks igniting a confrontation in the region that could yield to dire security consequences." Iraq's Foreign Ministry also called Haniyeh's killing "a threat to security and stability in the region." Oman also condemned the action.
Hamas allies in the region, in the meantime, rallied around the Palestinian resistance group.
"The martyrdom of leader Haniyeh ... will increase the determination and stubbornness of the mujahideen resistance fighters on all resistance fronts ... and will make their resolve stronger in confronting the Zionist enemy," Lebanese group Hezbollah, who are in a growing standoff with Israel, said in a statement.
The group described Haniyeh as "one of the great resistance leaders of our time who stood bravely against the American hegemony project and the Zionist occupation."
"Targeting him is a heinous terrorist crime and a flagrant violation of laws and ideal values," said Mohammed Ali al-Huthi, a political bureau member of Yemen's Houthi rebels. Afghanistan's Taliban rulers called Haniyeh's death "a great loss."
Pakistan, meanwhile, said the killing of the Hamas political leader was a "reckless act," describing his assassination as "terrorism."
"Pakistan views with serious concern the growing Israeli adventurism in the region. Its latest acts constitute a dangerous escalation in an already volatile region and undermine efforts for peace," Islamabad's Foreign Ministry said in a statement.
Malaysia also unequivocally condemned "all acts of violence, including targeted assassination," and urged all peace-loving nations to join in denouncing such acts.
"The incident underscores the urgent need for de-escalation and reinforces the necessity for all parties to engage in constructive dialogue and pursue peaceful resolutions," a government statement said.