Video has emerged from Syria which appears to show rebels executing regime loyalists in the embattled city of Aleppo.
The men, allegedly members of the Shabiha, or armed groups who have assisted in the government's crackdown, are lined up and shot at point blank range.
The narrator in the video, uploaded to YouTube, says the men are from the Barri clan, whom the rebels accuse of murder.
The development comes as fierce fighting rages for control of Syria's largest city, with rebel fighters putting up determined resistance to an army counter-offensive launched on Saturday.
Nationwide, at least 135 people were killed in violence on Wednesday - 74 civilians, 43 soldiers and 18 rebels, the Britain-based Syrian Observatory for Human Rights said.
On Tuesday, 124 people were killed nationwide, around half of them in Aleppo, it said.
Troops loyal to President Bashar al-Assad killed at least 35 people on Wednesday, mostly unarmed civilians, when they shelled and overran a suburb of the capital Damascus, residents and activist organisations said.
"The tanks and troops left around 4pm. When the streets were clear we found the bodies of at least 35 men," a resident, who gave his name as Fares, said by phone from Jdeidet Artouz, southwest of Damascus.
"Almost all of them were executed with bullets to their face, head and neck in homes, gardens and basements."
Fares, who did not give his last name for fear of retribution by Assad's forces, said soldiers from the Fourth Division, a praetorian guard unit under the command of Assad's brother Maher, came into his house.
"They examined my ID and let me go. They seem to have been looking for activists or young men with a certain profile to execute," he said.
He said the bodies were collected at the Omar bin al-Khattab mosque and buried in a mass grave dug by a bulldozer volunteered by the owner of nearby farmland.
"There are more bodies in al-Sahl area but we could not reach them because there is an army roadblock there," he said.
Another resident of the suburb said the total number of dead was at least 50.
Rami Abdelrahman, head of the SOHR, said 26 of 35 bodies his organisation tallied in Jdeidet Artouz have been identified and the victims were overwhelmingly civilians.
Air attacks on Aleppo
Separately, the UN said its military observers in Syria had seen the military use a fighter jet to strike the rebels in Aleppo, who were now armed with tanks.
"The observers now have confirmed information that the opposition is in possession of heavy weapons including tanks in Aleppo," Martin Nesirky, UN spokesman, said at the UN headquarters on Wednesday.
The UN observer mission also reported seeing a fighter jet attacking the city in a field visit to the city on Tuesday.
In-depth coverage of escalating violence across Syria
AFP correspondents on the ground have reported that rebels have captured a number of tanks, and some armoured units have defected with their vehicles.
Nesirky stressed that Ban Ki-moon, the UN chief, wants united international pressure on both sides in the civil war.
He said pressure should be brought to bear on "not just the Syrian government forces - who of course bear the lion's share of the responsibility for what is happening - but also on the opposition forces, to ensure that they do heed the calls, that they do stop the fighting".
The FSA's military chief in Aleppo, Colonel Abdel Jabbar al-Oqaidi, said the rebels had "thousands" of fighters in the city.
"The regime says it is fighting 'terrorist groups'. We tell the regime that we will chase them because they are the terrorists," Oqaidi told AFP.
"We will go after them in the whole of Aleppo, until the city is liberated."
Rebel commander Ferzat Abdel Nasser, an army general who defected a month ago, said: "The most important thing is to take over the intelligence branches. If these sites fall, victory is possible."
Kassem Saadeddine, FSA spokesman, said the rebels controlled half of Aleppo city and most of its province.
"We hope to create a safe zone in Aleppo and [the northwestern province of] Idlib," on the border with Turkey, Saadeddine said. Idlib and Aleppo are northern Syria's two main cities.
A safe zone would enable the rebels to bring in weapons more easily from nearby Turkey, and to set up a more organised military structure.
The World Food Programme said it had sent food assistance for distribution to 28,000 people in Aleppo over the next few days.
"The humanitarian situation is deteriorating in Aleppo and food needs are growing rapidly," the UN agency said.
The UN says that some 200,000 of the city's estimated 2.7 million population have fled their homes, many of them taking refuge in schools and other public buildings.
Government in exile
The FSA responded angrily to the announcement by a civilian dissident group in Cairo on Tuesday that it intended to set up a government in exile.
"This government in exile was stillborn because it was made by a single group that does not represent the whole of the opposition," the FSA spokesman told AFP.
Haytham al-Maleh, a veteran human rights activist, had said that he had been entrusted with forming a government-in exile, adding that he would consult "with the opposition inside and outside".
For his part, Assad said on Wednesday that the army was fighting for the nation's future.
He said the campaign to crush the uprising, which is now in its 17th month, was vital to Syria's future.
"The army is engaged in a crucial and heroic battle ... on which the destiny of the nation and its people rests," Assad said, in a speech carried by the official SANA news agency.
"The enemy is among us today, using agents to destabilise the country, the security of its citizens... and continues to exhaust our economic and scientific resources."
The US mocked Assad as "cowardly" for not delivering his speech to mark armed forces day in public.
"We think it's cowardly quite frankly to have a man who's hiding out of sight be exhorting his armed forces to continue to slaughter the civilians of his own country," Patrick Ventrell, a US state department spokesman, said.
"We think it's despicable to be exhorting his armed forces to continue this slaughter, and this bloodshed."
In another development on Wednesday, Lebanon deported 14 Syrians despite the violence over the border, drawing criticism from human rights activists.
The Lebanese authorities said the reasons for the expulsions were not political but a Human Rights Watch representative in Beirut said some of the deportees had expressed feared of persecution on their return.
"Fourteen men were deported to Syria today, despite the fact that four of them had asked not to be deported for fear of persecution if handed over to the Syrian authorities," the HRW representative told AFP.
One of them might be a political activist, the representative said, noting that the detainee had contacted HRW prior to being handed over to Syrian authorities at the border and expressed fear about what might happen to him.
But Lebanese General Security official told AFP that those deported were wanted for common law not political offences.
"These people were handed over to the Syrian authorities because they had problems with the judiciary and had committed crimes, and as far as we know they were not political activists," the official said.
"If they were, we would not have deported them."