A Free Syrian Army fighter stands in front of a building destroyed by a Syrian Air force air strike in the Haresta neighborhood of Damascus. (STRINGER/REUTERS)

The bodies of least 65 people shot in a mass killing were found in Aleppo on Tuesday, according to opposition activists.

A video posted online Tuesday showed many of the victims lying on the muddy banks of the Quweiq River in the Bustan al-Qasr neighborhood of southwestern Aleppo with their hands bound. Most appeared to have been shot in the head, and some of the victims appeared to be teenagers.

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 from rebel control.

Opposition activists said it was not clear who carried out the mass killing, when it happened or why. Since the bodies were fished out of the river, it was possible that the victims were shot somewhere outside the city, they said.

Some activists said the killing was probably carried out by the Syrian military or the pro-government shabiha militia and surmised that the victims could have been political detainees.

“We have a fear that they might be political prisoners from the central prison of Aleppo,” a reporter with the opposition Shaam News Network who goes by the name Majed Abdul Nour said in a Skype interview from Aleppo. “This river where we found them passes by the central prison.”

Opposition groups said they expected the number of dead to increase.

News of the massacre surfaced as President Obama pledged an additional $155 million in humanitarian aid for Syria on Tuesday.

“We’re under no illusions. The days ahead will continue to be very difficult,” Obama said in a statement. “But what’s clear is that the regime continues to weaken and lose control of territory.”

The announcement of increased aid comes a day before the International Pledging Conference for Syria is scheduled to be held in Kuwait. The conference, to be chaired by U.N. Secretary General Ban Ki-moon, aims to raise funds to address the dire humanitarian needs of Syrian civilians inside the country as well as the tens of thousands of refugees who have escaped the fighting.

In New York, meanwhile, Lakhdar Brahimi, the U.N. special envoy to Syria, prepared to deliver what was expected to be a grim report on the situation in the country to the U.N. Security Council on Tuesday.

Brahimi appeared to be making some progress late last month in talks with Russia, the Syrian government’s most powerful foreign ally. But his efforts were undercut by a speech by Syrian President Bashar al-Assad in early January that made it clear he would not leave power, one of the key demands of the opposition.

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.

The United Nations recently announced that at least 60,000 people have been killed in the bloody fighting in Syria since the uprising began in March 2011. But the international body’s inability to stop that killing has angered some opposition activists.

“The United Nations doesn’t do anything,” said the director of the Syrian Observatory for Human Rights, a monitoring group based in Britain, who uses the pseudonym Rami Abdulrahman. “They should investigate these bodies in Bustan al-Qasr. We gave them a lot of evidence of other killings, too, but they don’t act.”

In the video posted online Tuesday, the cameraman repeatedly mutters “God is great” while filming row after row of bodies in muddy clothes, many of them with pools of blood gathering beneath their heads on the banks of the Quweiq River. After a little more than three minutes of filming, shots ring out and the cameraman begins to run.

“The sniper is targeting me,” he says, before the video cuts off.

Ahmed Ramadan contributed to this report.