The bodies of at least 65 men, many of them with their hands bound behind their back, have been found on the muddy banks of a small river in the northern city of Aleppo, activists say.
The bodies, almost all of men in their 20s and 30s, were discovered Tuesday in the contested neighbourhood of Bustan al-Qasr, the director of the Britain-based Observatory for Human Rights, Rami Abdul-Rahman said.
Intense clashes between rebels and government troops have raged in the district since opposition forces launched an offensive on Aleppo in July.
Abdul-Rahman said it was not clear who was behind the killings, when they occurred or who the dead are.
Another activist group, the Local Co-ordination Committees, put the number of bodies found at 80 and blamed government forces for the killing.
There was no immediate comment from Syrian authorities on the bodies discovered Tuesday.
Since the Syrian uprising began in March 2011, there have been several cases of alleged mass killings. The rebels and the government routinely blame each other for the deaths.
An amateur video posted online showed dozens of bodies placed in rows on the ground and wrapped in blue blankets. A crowd of men, many of them covering their nose with a scarf, walk among the bodies, apparently trying to identify them.
A voice in the background says "number them," while another says "pray for them."
At one point, a man stops at a body and breaks down into tears, shouting: "He's my brother."
The video appeared genuine and corresponded to other AP reporting on the events depicted.
U.S. funds $155M in humanitarian aid
The violence came as U.S. President Barack Obama authorized an additional $155 million in humanitarian aid for the Syrian people Tuesday, as his administration grapples for a way to stem the violence without direct U.S. military involvement.
Obama announced the additional funding Tuesday in a video directed at the Syrian people.
"The relief we send doesn't say 'Made in America,' but make no mistake — our aid reflects the commitment of the American people," Obama said in the video, which was posted on the White House website.
The fresh funding brings the total U.S. humanitarian aid to Syria over two years to $365 million, according to the White House. Officials said the money was being used to immunize one million Syrian children, purchase winter supplies for a half million people, and to help alleviate food shortages.
The U.S. has long called for Syrian President Bashar al-Assad to leave power and says the fall of his regime is inevitable. In addition to the humanitarian aid, the White House has also ratcheted up economic sanctions on Assad's regime and recognized the rebel-led Syrian Opposition Council as the legitimate representative of the Syrian people.
Also, EU humanitarian aid commissioner Kristalina Georgieva said the EU committed another €100 million ($134 million) in help, bringing the overall EU total so far to €360 million.
Earlier in the day, Syrian rebels stormed a government intelligence complex in the oil-rich east of the country, freeing at least 11 people held in a prison at the facility, activists said.
After five days of heavy clashes around the intelligence compound in the city of Deir el-Zour, rebels finally overran the complex early Tuesday, the Observatory said. It was not immediately clear whether those freed from the compound's prison Tuesday were fighters or activists.
The activists said the compound was run by the Political Security Department, one of Syria's four most powerful intelligence agencies.
Amateur videos showed rebels raising an Islamic flag on top of the three-storey building as fighters carted away rifles and boxes of ammunition.
City under bombardment
Deir el-Zour has been the scene of heavy fighting since the uprising against Assad began in March 2011. The province, which goes by the same name as the city, is located along Syria's border with Iraq and includes several oil installations that the rebels have repeatedly targeted.
The aid group Doctors Without Borders said last month that government forces are shelling and bombing Deir el-Zour almost daily. It said tens of thousands of Syrians, many of them wounded, remain trapped in the city.
Also Tuesday, regime warplanes also carried several airstrikes on rebel positions in restive towns and villages around Damascus, including eastern Ghouta and Yalda, the Observatory said. The group relies on reports from activists on the ground.
After capturing several major army bases and government outposts, the rebels control large swathes of land in northeastern Syria. Assad's troops, however, continue to hold a tight grip on the capital after nearly two years of conflict.
The areas on the capital's doorstep have been rebel strongholds since early on in the revolt. In recent months, the rebels have used them as a base from which they have been trying to push into central Damascus, the seat of Assad's power.