UNITED NATIONS — The head of the UN’s humanitarian relief operations said Syria’s people — nearly seven years into a war that had claimed the lives of hundreds of thousands — still endured “a conflict with an atrocious, incalculable human cost.”

Stephen O’Brien, the UN’s undersecretary-general for humanitarian affairs, addressed the UN Security Council at a meeting Wednesday to update ambassadors on the deteriorating humanitarian crisis in pockets of Syria. It is the Syrian people who are paying a high and tangible price for their leaders’ political failure, he said.

“Month after month it is the unremitting fearful plight of the Syrian people which sears into our hearts and outrages and torments our minds,” he said. As many as 500,000 people have been killed since war broke out in March 2011, O’Brien said. “Surely we can do better.”

The 15-member Security Council itself has often deadlocked on the Syrian question, with some of the five veto-wielding countries invoking the measure to block resolutions that experts have said would have helped end the conflict. Russia and China, in particular, have vetoed several measures that they said were overreaching and would have given forces — such as the United States — license to remove Syrian President Bashar Assad from power.

O’Brien detailed barbaric acts committed by military forces as well as the foreign fighters — Islamic State and Al Qaeda affiliates, to name few — bent on the overthrow of Assad, including the bombardment of medical facilities, the rape, torture and sexual enslavement of Yazidi women and the tossing off buildings and subsequent stoning to death of homosexuals.

The conflict has sparked an exodus of millions of Syrians into neighboring Jordan, Turkey and Lebanon and European countries, with refugees walking hundreds of miles and braving the Mediterranean in unseaworthy boats and rafts in desperate attempts to flee the war.

He spoke as the United Nations Special Adviser on the Prevention of Genocide, Adama Dieng, reported that as many as 25,000 civilians in Raqqa, the city where Islamic State has used as a headquarters in Syria, remained trapped as the Syrian military forces, backed by Russia, and the rebel groups and coalition backed by the United States, pummel the city to rout out the remaining Islamic State fighters.

“The city is under intense bombardment by international counter-ISIL coalition forces and the Syrian Democratic Forces,” he said, using an acronym for Islamic State. “In addition, ISIL is reportedly using civilians as human shields . . . In addition to the terrible situation they face in the city, ISIL is reportedly killing those who try to escape, and coalition forces are targeting boats on the Euphrates River, which had been one of the remaining escape routes for civilians.”

Diplomacy thus far has not yielded a halt to the conflict, which began as the Arab Spring movement against authoritarian regimes spread across northern Africa and the Middle East. Several longtime leaders of Tunisia, Egypt and Libya, to name few, stepped down or were removed.

Assad’s response to the nonviolent demonstrations for his ouster was a military crackdown, experts said, which sparked military resistance from dozens of rebel groups and the infusion of thousands of foreign fighters pledging allegiance to Islamic State and other jihadi groups.

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The UN’s Special envoy for Syria, Staffan De Mistura, has coordinated seven meetings in Geneva and attended parallel meetings in Astana, Kazakhstan, that are designed to broker cease-fires and a de-escalation of hostilities in particularly treacherous spots in the Syrian war. Other peace-seeking sessions have taken place in Moscow, Paris, Tehran and Riyadh over the years. The Astana and Geneva meetings resume in September and October, respectively.

“This is a time for realism and focus, for shifting from the logic of war to that of negotiation, and for putting the interests of the Syrian people first,” de Mistura told the Security Council on Wednesday. “If I could identify one thing above all that can make the difference, it will be a sense of unity of purpose internationally with clear priorities and common goals.”

O’Brien, who was addressing the Security Council for the last time as head of the Office for the Coordination of Humanitarian Affairs, made an emotional appeal to the ambassadors to stop the conflict.

“As I survey this famous horseshoe table where you, the 15 members of the Security Council representing the world’s hopes for international peace and security are charged as you are, even while representing your own capitals and their respective competing perspectives to rise above the 15 interests to reach the collective responsibility of the interests of all the peoples of the world,” O’Brien said.