The bodies of some 65 young men, apparently shot execution-style, were found in the northern Syrian city of Aleppo, opposition activists said Tuesday, DPA reported.
In New York, the UN-Arab League envoy for Syria, Lakhdar Brahimi, briefed the UN Security Council on his months-long diplomatic shuttle between capitals to try to work out a political solution to the conflict.
Before Brahimi met behind closed doors with the UN council, Secretary General Ban Ki-moon and his Arab League counterpart Nabil Elaraby held talks in Cairo on the Syrian conflict, which they said has created a dire humanitarian crisis.
"They reiterated their strong support for Lakhdar Brahimi ... and noted with concern the dire humanitarian situation in Syria, ahead of Wednesday's donor conference in Kuwait," a UN statement said.
Video on YouTube showed a row of bodies along the banks of the river; many had their hands tied. The videos' content could not be independently verified.
Observatory chief Rami Abdel-Rahman said he had no information on who was behind the killings and that the number could reach 80, as more bodies were feared to be under water.
Abu Omar al-Halabi, a Free Syrian Army commander in the city, told dpa said that most of the dead were reported kidnapped earlier this year.
Relatives were gathering at the banks of river looking for their missing sons, activists in the area said.
One video circulating on opposition websites showed dozens of men gathering at the site, some wearing medical gloves, others carrying machine guns.
"Their only crime was that they were residents of Bustan al-Qasr and they were Sunni Muslims," a man was shown shouting as he looked down at bodies awaiting identification.
Syrian television station al-Dunia, which strongly supports President Bashar al-Assad, accused "terrorists" of the killings.
In Ras al-Ain, a town on the Turkish border, clashes between rebels and a Kurdish militia ended with the Kurds taking control of a number of rebel-held positions, the Observatory said.
The People's Defence Committees, a militia linked to the Kurdish Democratic Union Party (PYD), have been fighting rebels there since mid-January.
Rebels suspect the PYD of collaborating with the al-Assad regime, while the PYD says it is defending the town from Islamist extremists operating at the behest of Turkey.
The continued violence came as the United Nations warned that its resources were not adequate to deliver assistance to Syrians in need, ahead of a donors conference in Kuwait on Wednesday that aims to secure more than 1.5 billion dollars in new cash pledges.
According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), the Syria Humanitarian Response Plan requires more than 519 million dollars to help some 4 million people inside Syria, including an estimated 2 million who are internally displaced.
Another 1 billion dollars is needed to support more than half a million refugees who have fled to Jordan, Iraq, Lebanon, Turkey and Egypt, it said.
In Brussels, the European Union said it would pledge 100 million euros (135 million dollars) of humanitarian aid at the donors conference, in addition to 100 million euros given last year.
US President Barack Obama on Tuesday promised an additional 155 million dollars, bolstering US aid to 365 million dollars.
In Kuwait, non-governmental charity organizations pledged 183 million dollars for Syrians affected by the 22-month conflict, the official Kuwaiti News Agency KUNA said.