Human rights activists have condemned the public shooting in Syria of four apparent Assad loyalists by rebels in the battleground city of Aleppo.
Video posted online shows the men, who included the alleged head of a feared local militia, being put up against a wall and shot with Kalashnikov rifles.
Human Rights Watch has told the BBC the act was potentially a war crime.
Syrian President Bashar al-Assad has said the nation's fate is being decided by the fight against the rebels.
The head of the UK-based Syrian Observatory for Human Rights, which documents violence by all sides in the conflict, condemned the killings by the rebels as "criminal".
But Russia, one of Syria's closest allies, said the video showed there were human rights violations on both sides.
Government forces have been battling to oust Free Syria Army (FSA) rebels from Aleppo, the country's biggest city and its commercial capital, since they launched an assault last month.
The rebels appear to control large parts of the city despite government assertions that they have suffered heavy losses and are being mopped up by security forces.
The UN mission in Syria reports that the rebels in Aleppo are now armed with some heavy weapons including tanks and says that helicopters, heavy machine guns and artillery are also being used in the fighting.
"We are calling on all parties to exercise utmost restraint... to distinguish between civilians and [combatants] in this conflict," said spokeswoman Sausan Ghosheh.
Activists say at least 135 people were killed across Syria on Wednesday, in figures which could not be confirmed independently.
The public shootings seem to have taken place on Tuesday, in what looks like a schoolyard.
In a video which appeared on YouTube, gunmen can be seen leading a number of men in their underwear, some of them bruised or bloodied, into the yard, which is crowded with men shouting religious slogans.
Chants include, "The Free Syrian Army forever, we'll trample on the head of Assad".
After the half-naked men are put up against a wall, the camera moves back behind the crowd, losing sight of them.
Heavy gunfire from Kalashnikov assault rifles erupts, after which the camera shows a pile of bodies by the wall.
One of the men killed has been identified as Ali Zeineddin al-Berri, known as Zeno, accused of leading a pro-regime shabiha militia group which killed 15 FSA fighters during a truce in Aleppo on Tuesday.
Clive Baldwin, a senior legal adviser for Human Rights Watch (HRW), told the BBC: "What it looks like is execution of detainees and if that is the case, that would be a war crime."
HRW, he added, had been seeing evidence of abuses such as torture and executions by rebel elements "for some time".
Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights, said of the footage: "This is criminal. This is revenge."
Russian Deputy Foreign Minister Gennady Gatilov said the killings confirmed human rights violations were "taking place on both sides".
The Russian President, Vladimir Putin, is expected to discuss Syria when he meets UK Prime Minister David Cameron on a visit to in London on Thursday.
The Aleppo video clearly shows that large numbers of rebel fighters retain control of several parts of Aleppo and seem even to be expanding, despite reports on the state media that security forces are prevailing, says the BBC's Jim Muir, in neighbouring Lebanon.
Mr Assad has not spoken in public for two weeks. On Wednesday, he issued a written statement marking armed forces day.
He praised soldiers for confronting "armed terrorist gangs", saying: "The fate of our people and our nation, past, present and future, depends on this battle."
The US accused Mr Assad of cowardice for "exhorting his armed forces to continue to slaughter the civilians of his own country" while "hiding out of sight".
Amnesty International says government forces committed crimes against humanity this month in Aleppo.
In the report, based on research carried out in May, the rights group appealed to the UN Security Council to refer Syria to the International Criminal Court and impose an arms embargo on the country.
Amnesty accuses security forces and the shabiha of firing on peaceful protesters and bystanders, including children.
It also says medical teams were targeted and those arrested were often tortured.
The UK-based Syrian Observatory for Human Rights reported 135 deaths on Wednesday while the Local Coordination Committees gave a figure of 170.
Activists estimate some 20,000 people have died since March last year.