The Syrian Observatory for Human Rights, an activist organization based in Britain with a network of contacts in Syria, said at least 50 bodies had been located, some scattered along the banks of a small river in the Bustan al-Kaser neighborhood, which is mostly under rebel control. Later reports put the tally at 80.
“This is another new massacre that has been committed in Syria, adding to the constant massacres that have been occurring, while the world watches silently and the international and Arab community are being hypocrites,” the Syrian Observatory said in statement.
The video emerged as the United Nations reported a fresh upsurge in the number of refugees known officially to have fled Syria, increasing the total in neighboring countries to more than 700,000 from 500,000 in December.
At the same time, rebel fighters seeking Mr. Assad’s overthrow appeared to have made advances in the east of the country, raiding a security office in the eastern city of Deir al-Zour, where government forces have seemed to reduce their presence to concentrate on the center, giving rebels more freedom to maneuver and in some cases siphon fuel from gas and oil fields there.
Activists said the insurgents in Deir al-Zour included Islamist fighters from the Al Nusra Front, which the United States regards as a terrorist organization. The rebels freed 11 detainees and captured a tank and three armed personnel carriers, according to the Local Coordination Committees, an anti-Assad activist network in Syria.
In Aleppo, Syria’s most populous city where rebels and government forces are locked in a contest for control, video on YouTube — which was not independently verifiable — showed the shadow of a cameraman moving from one corpse to the next, briefly halting at each. Many appeared to have their hands bound behind their backs. Another video showed five bodies jammed into what looked like a metal container or the back of a small truck with more corpses lined up on the street outside and yet more on the flatbed of a pickup.
Video posted later showed what seemed to be another set of corpses, some of them older men with their arms bound in front of them. The wounds shown in the footage suggested that they had been shot in the temple or the chest.
Crowds of civilians milled around, some wearing blue surgical gloves.
It was not clear when the men had died or who they were. One man said the killers had chosen their victims because they were Sunni Muslims.
The SANA state news agency made no immediate reference to the apparent mass killings.
The Syria conflict began in March 2011 as a peaceful protest but has since spiraled into civil war.
In Geneva, the United Nations High Commissioner for Refugees said on Tuesday that there had been an “unrelenting flow of refugees” across Syria’s borders, principally into Jordan, Lebanon, Iraq and Turkey.
The highest numbers were in Turkey, Jordan and Lebanon but smaller numbers had been registered in Egypt and North Africa, said Sybella Wilkes, a spokeswoman for the refugee agency.
The total now exceeds 700,000, made up of around 580,000 registered and the rest waiting to be registered as refugees. The spurt in refugees meant that 200,000 have fled in less than two months since early December when the total was around 500,000.
“We are trying to clear a backlog of people because the numbers have gone up so dramatically,” in Jordan and Lebanon particularly, Ms. Wilkes said.
The fighting has long ceased to be a straight contest between government and rebel forces. In the northern town of Ras al-Ain on the border with Turkey, rebels have fought Kurds. and in Deir al-Zour, rivalries among the groups claiming to have overrun the security office showed the contest between them to attract arms and recruits.
Omar Abu Layla, an activist documenting the fighting, said local and Al Nusra groups had joined in the fighting.
“Al Nusra are good in suicide attacks, but our battalions are better than them at storming,” he said.
In the central city of Homs, meanwhile, the toll of the fighting among the dwindling number of inhabitants seemed evident on Tuesday as government forces launched a rocket attack on the Jouret al-Shiyah neighborhood.
“Mercy, dear God, Mercy. I don’t know what’s going on. I feel that they’re shelling right above us,” said Um Abdo, a resident in her fifties. “I feel that the whole world is shaking. The shelling is so heavy and so close. Pray for us please. I swear we are drained and exhausted.”
Hania Mourtada reported from Beirut and Alan Cowell from London. Hala Droubi contributed from Dubai, United Arab Emirates, and Anne Barnard and Hwaida Saad from Beirut.