The comments from Gambian Foreign Minister Mamadou Tangara came as a senior Iranian official said during the same meeting that the Islamic republic would need to defend itself from Israel, which it blames for Haniyeh's death last week in Tehran.
Iranian and Palestinian officials called for Wednesday's gathering of the 57-member Organisation of Islamic Cooperation (OIC) in the Saudi coastal city of Jeddah, saying the body needed to respond to the killing of the Hamas leader.
"This heinous act serves only to escalate the existing tensions potentially leading to a wider conflict that could involve the entire region," said Tangara, whose country currently chairs the OIC.
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." Israel has not commented on Haniyeh's death, which Iran has vowed to avenge, putting the region on edge.
"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," Ali Bagheri, Iran's acting foreign minister, told the OIC on Wednesday.
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 1,198 people, mostly civilians, according to an AFP tally based on Israeli official figures. 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 Hamas-run territory's health ministry, which does not give details of civilian and militant deaths.
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.
The strong statement masked divisions within the assembled group, as some countries proposed threatening to disrupt oil supplies to Israel and its allies as well as severing any economic and diplomatic ties.
Diplomats said at the time that countries that have formal diplomatic ties with Israel, including the United Arab Emirates and Bahrain, came out against the idea.
Saudi political analyst Mohammed bin Saleh al-Harbi told AFP that, for Wednesday's OIC meeting, "we cannot expect more than condemnation and denunciation."