Top Muslim diplomats on Wednesday said Israel was “fully responsible” for the “heinous” killing of Hamas political leader Ismail Haniyeh in Iran and warned it could destabilize the region.

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 the attack on Haniyeh, setting the Middle East on edge.

Israel has not commented on the death of Haniyeh, who lived in Qatar and was a major player in talks to end the war in the Gaza Strip.

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,” which it described as “a serious infringement” of Iran’s sovereignty.

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.”

Iran’s acting Foreign Minister Ali Bagheri-Kani 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 defense 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.”

Hamas’s Lebanese ally Hezbollah has also pledged to retaliate for Haniyeh’s killing and that of its military commander Fuad Shukr in an Israeli strike in Beirut hours earlier.

Wednesday’s meeting was far from the first time the bloc has weighed in on the war, which began with Hamas’s October 7 attacks on southern Israel.

That operation resulted in the deaths of about 1,200 people, according to Israeli tallies.

Palestinian militants seized 251 hostages, 111 of whom are still held in Gaza, including 39 the Israeli military says are dead.

Israel’s retaliatory military campaign in Gaza has killed at least 39,677 people, according to the Gaza health ministry.

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.