United Nations Warns Syrian Rebels Over Atrocities BEIRUT, Lebanon - The top United Nations human rights official warned opposition fighters in Syria on Monday that they would not be immune from prosecution for atrocities, as videos from the Syrian city of Aleppo appeared to show a mass execution by rebel fighters of bound and blindfolded Syrian government soldiers. One of the videos, first publicized on Monday on the Brown Moses blog, which curates and analyzes video evidence from Syria, showed at least 20 corpses lying in crooked row on a bloodstained street curb. The victims wore fatigues but no shoes. Several appeared to have been shot in the head. In that video and another that captured the same scene, different rebel groups appear to take responsibility for the killings. It was impossible to immediately confirm the authenticity of the videos, or to determine exactly when and where they were recorded. If confirmed, the executions were likely to add to growing concerns about the conduct of the militias fighting to topple the Syrian president, Bashar al-Assad, and particularly their treatment of prisoners. In a brutal episode in late July, a group of rebel fighters were seen in a video executing several captives - members of an Aleppo family accused of being enforcers for the government - with a spray of gunfire. In recent days, other videos have captured summary executions by the rebels. Speaking in Geneva on Monday, Navi Pillay, the United Nations high commissioner for human rights, warned of atrocities by both the government and its opponents. Both, she said, "deploy snipers that target civilians." Ms. Pillay also said the Syrian government's attacks on civilians and destruction of homes "may constitute war crimes or crimes against humanity," according to a transcript of Ms. Pillay's remarks on her office's Web site. And in a stern warning directed at antigovernment forces, Ms. Pillay noted the "undoubted climb in human rights violations" attributed to the rebels, including abductions and summary executions. "Opposition forces should be under no illusion that they will be immune from prosecution," she said. In one of the videos showing the executed soldiers, a narrator claims that a rebel battalion called Salam al-Farisi was responsible for killing the men. In a subsequent video posted by the London-based Syrian Observatory for Human Rights, a man attributes responsibility to a different battalion. "Assad's dogs," the man says, panning the camera across the scene, of bodies contorted in anguish or slumped, in a fetal position. God is great. The leader of the Syrian Observatory, who uses the pseudonym Rami Abdul-Rahman for safety reasons, said the exact location of the killings was not clear, but that the soldiers may have been part of a contingent from a military base in the Hanano district that rebel fighters attacked on Friday. Also on Monday, Egypt's Foreign Ministry announced that it was convening a meeting of regional states to try to find a peaceful resolution to the Syrian conflict. Diplomats from Turkey, Iran, Saudi Arabia and Egypt are expected to meet in Cairo in the coming days, in an initiative has been widely viewed as an effort by Egypt's new president, Mohamed Morsi, to reassert his country's regional leadership role. The prospects for that effort seemed questionable, at best: while Iran has been a strong supporter of Mr. Assad's government, Mr. Morsi has called for the Syrian president to step down. Saudi Arabia and Turkey have actively helped Mr. Assad's opponents, with weapons and other logistical support. In a statement, the Egyptian Foreign Ministry said that the meeting would work toward the goals of ending violence, preserving Syria's territorial integrity, rejecting foreign military intervention and starting a political process that could achieve a "democratic, pluralist system." Hala Droubi contributed reporting from Dubai, United Arab Emirates, and David D. Kirkpatrick from Cairo.