The video, posted on YouTube yesterday, shows 10 men wearing t-shirts and camouflage trousers lying face down next to a building and a lookout tower. Even before the shooting, two of the men are not moving and one has blood coming from his torso.
"I swear to God that we are peaceful," begs one of the men to the camera, which is being held by the gunman. Cowering, the man gets up to plead with rebels. As he approaches a rebel off-screen, a shot is heard and he returns holding his bloodied arm.
The cameraman then points the camera along the barrel of his Kalashnikov assault rifle as he shoots the men.
"God is great. Jabhat al-Nusra," he says, referring to the secretive al-Nusra Front, an Islamist rebel unit linked to al Qaeda that has claimed responsibility for several suicide bomb attacks around the country.
The gunman gets on the back of a pickup truck and the camera pans to show the man who had been shot in the arm still moving. More shots are fired and his body spasms.
Reuters could not immediately verify the authenticity of the footage. Comments accompanying the video said it was filmed in Ras al-Ain, a town on the border with Turkey where pitched battles have raged in recent weeks.
Human Rights Watch officials said they were also investigating the video, but could not yet confirm its veracity.
"It certainly is a very disturbing video, and it is important to establish what happened," said Peter Bouckaert, emergencies director at the New York-based group.
Syria's uprising started with peaceful protests which were harshly suppressed by troops and has evolved into a civil war in which foreign jihadi fighters have joined ranks with defecting soldiers and armed civilians.
The 20-month-old revolt against President Bashar al-Assad has left 40,000 people dead. World powers who support the uprising say they are wary of providing arms to rebel groups due to the increasing role of Islamist radicals.
Rights groups accuse both rebel groups and government forces of war crimes including summary executions and torture.