Horrific videos purportedly showing Syrian rebels throwing the bodies of postal workers off a roof and a man's throat being savagely cut triggered outrage among rights activist on Monday.
Three videos all showing the apparent atrocities in the province of Aleppo, including a bound man being repeatedly shot, were posted on YouTube on Monday but their authenticity could not immediately be verified.
Both sides in the 17-month conflict have been accused of human rights violations as reports of cold-blooded killings mount.
"What is the difference between them and a wild animal in the jungle? At least a wild animal does not kill unless it is hungry," said Massoud Akko, a Kurdish activist and co-founder of the Association of Syrian Journalists.
Syrian Observatory for Human Rights director Rami Abdel Rahman said he strongly condemned such atrocities, whoever was behind them, if the videos were confirmed.
In one grisly scene, a crowd of people shouting "Allahu Akbar" (God is greatest) gathered around several bodies crumpled on the ground outside a building before another three victims are hurled one-by-one from the rooftop.
The incident was said to have taken place in rebel-controlled Al-Bab near the northern metropolis of Aleppo and the victims were identified as postal workers, but it was not clear when the killings occurred.
"These are the heroes of Al-Bab city who are inside the post office," the man shooting the video said.
When the body of one man was thrown to the ground, the crowd is heard shouting: "This is a shabiha," referring to the pro-government militia.
In another shocking amateur video, a blindfolded man, with his hands tied behind his back, struggled as a group of men forced him to lie down on a pavement in Aleppo.
The man calls out: "I would rather die by a bullet."
A man retorts: "Shut up."
As the group chanted "Allahu Akbar," the assailant forced what appeared to be a small knife repeatedly across his throat as his blood spurted onto the pavement.
"This is the fate of all the shabiha and those who support Bashar (al-Assad)," said the man filming the video.
And a third clip, purportedly shot in Aazaz, also in Aleppo province, showed a bearded man being hauled out of a car boot with his hands tied behind his back and pushed to the ground.
One man opens fire on him with a small pistol, only to be joined by another with a rifle. They shoot many times at the man, who dies face down in a field.
"If these videos are confirmed, such atrocities harm the revolution. They only benefit the regime and the enemies of the revolution," Abdel Rahman told AFP.
Online activists also condemned the killings, which highlight the escalating brutality of a conflict that started out as a peaceful uprising but which has deteriorated into a brutal civil war. The Observatory says at least 21,000 people have been killed countrywide since March last year.
"What is the difference between them and those who kill your children, women and men? This is not a justified reaction at all!" said Akko.
Brothers Mohammed and Ahmed Malas, both dissident artists who live in exile, posted on their Facebook page: "You are killing in the name of God, it seems, but you are killing just like Assad kills. You do not know God."
In recent weeks, activists have frequently expressed concern about human rights abuses by the rebels, as well as repeated atrocities by the regime since it launched its brutal crackdown.
One protester in Aleppo on August 10 held up a poster reading: "Correcting the mistakes of the revolution and its path is necessary for its victory."
Another poster, held up by a protester in Aleppo's Bustan al-Qasr district, read: "O sweet Free Syrian Army, don't take me prisoner at your checkpoint."
On August 8, activists and Free Syrian Army commanders distributed an 11-point code of conduct signed by scores of brigade commanders and rebel leaders.
"I swear to my people and the revolution that I will not engage in any practice that undermines the principles of our revolution: the principles of freedom, citizenship and dignity," reads one article.
"I will respect human rights... our tolerant religious principles and international human rights law -- the same human rights that we are struggling for today.