The declaration came at the end of an extraordinary meeting of the Saudi-based Organisation of Islamic Cooperation (OIC) called in part by Iran, which has vowed to retaliate for Haniyeh's assassination, setting the Middle East on edge.

Israel has not publicly commented on the death of Haniyeh. However, it informed US officials that it was behind the assassination of Hamas leader, immediately after Iran announced his death on 31 July, the Washington Post reported.

After foreign ministers gathered at the OIC's headquarters in the Saudi coastal city of Jeddah, the bloc issued a statement saying it "holds Israel, the illegal occupying power, fully responsible for this heinous attack", adding that it was "a serious infringement" of Iran's sovereignty.

The deputy foreign minister for Saudi Arabia, which until Wednesday had not commented on Haniyeh's assassination in Tehran, described it in similar terms, according to a Saudi government statement.

During the opening ceremony, Mamadou Tangara, foreign minister for current OIC chair The Gambia, said Haniyeh's death risked deepening and widening the ongoing bloodshed in the Middle East.

"This heinous act serves only to escalate the existing tensions potentially leading to a wider conflict that could involve the entire region," Tangara said.

Haniyeh's killing "will not quell the Palestinian cause but rather it amplifies it, underscoring the urgency for justice and human rights for the Palestinian people," he said.

"The sovereignty and territorial integrity of nation states are fundamental principles underpinning the international order.

"Respecting these principles has profound implications and their violation equally carries significant consequences."

Escalation fears
 

Iran's acting foreign minister, Ali Bagheri, reiterated Tehran's view that it needs to respond.

"Currently, in the absence of any appropriate action by the (United Nations) Security Council against the aggressions and violations of the Israeli regime, the Islamic Republic of Iran has no choice but to use its inherent right to legitimate defence against the aggressions of this regime," he said.

US State Department spokesman Matthew Miller voiced hope on Wednesday that "all parties that have a relationship with Iran impress upon Iran, the same way we've been impressing upon the government of Israel, that they shouldn't take any steps to escalate the conflict".

Miller said the United States had been in touch with a number of nations attending the OIC meeting and believed there was a "broad consensus" that "escalation would only exacerbate the problems facing the region".

Lebanon's Hezbollah has also pledged to retaliate for Israel's killing of Haniyeh and that of its military commander Fuad Shukr who was killed in an Israeli strike in Beirut hours earlier.

'Condemnation and denunciation'
 

Wednesday's meeting was far from the first time the bloc has weighed in on Israel's war on Gaza which has decimated the Palestinian territory.

In addition to issuing regular statements condemning civilian deaths in Gaza, OIC leaders gathered with their counterparts from the Arab League in November for a summit that condemned Israeli forces' "barbaric" actions in Gaza.

Israel's relentless bombardment and ground invasion have killed at least 39,677 Palestinians in Gaza, many of them women and children, according to the territory's health ministry.

The brutal assault has also wounded more than 91,645 Palestinians, according to Gaza's health ministry, with thousands more presumed dead or missing under the rubble.

*This story was edited by Ahram Online.