Ismail Haniyeh arrives at the Iranian parliament to attend the swearing-in ceremony of President Pezeshkian. AP

Palestinian president Mahmud Abbas said on Wednesday the killing of Hamas chief Ismail Haniyeh was a "cowardly act" and urged Palestinians to remain united against Israel.

"President Mahmud 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."

Turkish President Recep Tayyip Erdogan on Wednesday condemned the "perfidious assassination" in Tehran of his close ally and "brother" Ismail Haniyeh, the political leader of Palestinian movement Hamas.

"May God have mercy on my brother Ismail Haniyeh, fallen in martyrdom after this odious attack," Erdogan wrote on the X social media platform, denouncing "Zionist barbarity".

Haniyeh was based in Qatar, which has been a mediator in the Gaza conflict, but also spent time in Turkey after going into exile in 2017. Qatar's foreign ministry called the killing a "heinous crime" and "shameful assassination".

The ministry said the strike in Tehran was a "dangerous escalation" and "a flagrant violation of international and humanitarian law".

"This assassination and the reckless Israeli behaviour of continuously targeting civilians in Gaza will lead to the region slipping into chaos and undermine the chances of peace."

Former Iranian president Ebrahim Raisi meets Ismail Haniyeh in Tehran. File/AFP

Turkey echoed the condemnation. "We condemn the assassination of the leader of Hamas's political office, Ismail Haniyeh, in a shameful assassination in Tehran," the foreign ministry said, adding that "this attack also aims to spread the Gaza war to a regional dimension".

"Once again the (Benjamin) Netanyahu government has shown that it has no intention of achieving peace," the ministry said. "If the international community does not take measures to stop Israel, our region will face much larger conflicts."

Syria

Syria's foreign ministry condemned the killing of Haniyeh, blaming Israel and warning that the latest escalation could "set the entire region ablaze".

"Syria condemns this blatant Zionist aggression," the ministry said, describing Haniyeh's killing as a "despicable act" and adding it "considers that the continued disregard of international laws by the Israeli entity... may set the entire region ablaze".

Russia and China

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.

Turkish President Recep Tayyip Erdogan with Ismail Haniyeh in Istanbul. File/AFP

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."

"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.

It 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 member of the political bureau of Yemen's Houthi rebels who have staged rocket attacks on Israel.

Afghanistan's Taliban rulers called Haniyeh's death "a great loss".

Haniyeh was assassinated in the early hours of the morning in Iran, the Palestinian group said, drawing fears of wider escalation in a region shaken by Israel's war in Gaza and a worsening conflict on the Israel-Lebanon border.

Agencies