At least 65 people were found dead Tuesday in the contested Syrian city of Aleppo, many of them bound and shot execution-style, according to opposition activists. The discovery served as a grim reminder of the country’s bloody descent into violence and the failure of diplomatic efforts to stop the killing.

Opposition activists said it was not clear who had carried out the killings, when they happened or why. Some activists said they believed that the Syrian military or a pro-government militia was responsible and that the victims may have been political detainees.

The killings provided a sobering backdrop to stalled diplomatic efforts to end the fighting. Lakhdar Brahimi, the U.N.-Arab League envoy to Syria, issued an impassioned appeal to members of the U.N. Security Council, asking them to put aside their differences and take firmer action to help stop the bloodshed.

Syria, Brahimi warned, was on the verge of disintegrating.

“I’m sorry if I sound like an old broken record,” Brahimi told the council during a closed session, according to notes of his briefing obtained by The Washington Post. “The country is breaking up before everyone’s eyes.”

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

Brahimi told the council that the effort to persuade the warring factions to enter political talks had run aground, with the Syrian government and the armed opposition unwilling to talk to each other. Meanwhile, key regional powers had picked sides in the conflict, he said, transforming Syria into a “playground for competing forces.”

President Obama, who has been under political pressure to do more to stem the violence, announced Tuesday that the United States will provide $155 million in new humanitarian assistance to Syrians, nearly doubling the American commitment. Obama said the assistance would go toward providing medicine, food, and clothing and blankets as winter approaches.

Obama made his pledge on the eve of a donors’ conference in Kuwait sponsored by the United Nations. U.N. officials warned Monday of increasingly desperate conditions for about 4 million Syrians inside the war-torn country and about 650,000 in refu­gee camps outside. The officials said the United Nations has received pledges for only a small percentage of the $1.5 billion it is seeking to provide relief.

In Syria, meanwhile, opposition groups said they expected the number of dead in the Aleppo killings to increase.

A video posted online Tuesday showed many bodies — apparently including those of some teenagers — lying on the muddy banks of the Quweiq River in the rebel-held Bustan al-Qasr neighborhood of southwestern Aleppo. Bustan al-Qasr has been the site of heavy fighting in recent days as the Syrian military has launched several attacks to retake the neighborhood.

Deghanpisheh reported from Beirut. Ahmed Ramadan in Beirut contributed to this report.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.