Gaza: Palestinian President Mahmoud Abbas, leader of the Fatah party, strongly condemned the attack according to Palestinian media.
"President Mahmoud Abbas of the State of Palestine strongly condemned the assassination of Hamas leader Ismail Haniyeh, deeming it a cowardly act and a serious escalation," Abbas's office said in a statement.
"He urged our people and their forces to unite, remain patient, and stand firm against the Israeli occupation."
Fatah and Hamas were longtime rivals before they signed a unity accord in China earlier this month. Abbas had removed Haniyeh from his office as Palestinian prime minister in 2007, though Haniyeh refused to acknowledge his decree for years.West Bank factions calls for strike
Palestinian factions in the occupied West Bank have called for a general strike in the wake of Haniyeh's killing.
"The national and Islamic factions in Palestine announce a comprehensive strike and anger marches to [protest] the assassination of the great national leader Ismail Haniyeh, which came in the framework of Zionist state terrorism and its war of extermination," they said in a joint statement.
While Hamas's control was mostly limited to the Gaza Strip, it had support from other militant groups in the West Bank and recently signed a unity deal agreement with other factions including rival party Fatah.Iran's Supreme Leader Ayatollah Ali Khamenei vowed that "harsh punishment" awaited Israel after an airstrike killed the leader of Hamas, Ismail Haniyeh.
"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 cited by official news agency IRNA.
At the same time, the country's new President Masoud Pezeshkian paid tribute to Haniyeh on social media site X, writing, "yesterday I raised his hand and today I have to bury him."
Haniyeh had been in Tehran to attend Pezeshkian's inauguration.
Without mentioning Israel by name, Pezeshkian further wrote that those responsible would "regret its cowardly act," and that "the two proud nations of Iran and Palestine will be stronger than before."
Qatar condemns 'heinous crime' after killing of Hamas leader
Qatar's Foreign Ministry said that the death of Haniyeh was a "heinous crime." Like other regional governments, Doha also warned that the strike in Iran was a "dangerous escalation" of growing tensions across the Middle East.
The statement continued, "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."
"The ministry reiterates the State of Qatar's firm position rejecting violence, terrorism and criminal acts, including political assassinations, regardless of the motives and reasons," it added.
Much of Hamas's political leadership, including Haniyeh, live in exile in Qatar, which has hosted them with the blessing of the United States since 2012.
China voices concern over conflict escalation
Beijing also joined the chorus voicing worries that the attack that killed Hamas leader Ismail Haniyeh in Tehran marked a dangerous escalation in regional tensions that could lead to all-out war.
"We are highly concerned about the incident and firmly oppose and condemn the assassination," Foreign Ministry spokesman Lin Jian said.
"We are deeply concerned that this incident may lead to further instability in the regional situation," he added.
China has seen itself as a peace broker in the conflict, offering to host both Palestinian and Israeli leaders for cease-fire talks. It also played host to a recent summit in which Hamas, which has controlled Gaza, and Fatah, which is in power in the occupied West Bank, put over a decade of conflict to rest and signed a unity deal.