Israel has not commented on the death but many countries have warned that it will have a major impact on efforts to bring a ceasefire in the Gaza war. Here are the key reactions:

-- Qatar,Turkey and Syria warn of chaos

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".

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 said Hamas chief killing could 'set the region ablaze'

-- Russia and 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."

-- Resistance allies rally

Hamas allies in the region rallied around the Palestinian resistance.

"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 resistance movement 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 member of the political bureau of Yemen's Houthis who have staged rocket attacks on Israel. Afghanistan's Taliban rulers called Haniyeh's death "a great loss".