Rebels from the jihadist Al-Nusra Front executed three captured soldiers in the eastern city of Deir Ezzor, the Syrian Observatory for Human Rights reported on Tuesday.
"Al-Nusra Front executed three regime troops who were captured on January 5 from the technical services building in Deir Ezzor city," the watchdog said, adding that it was not clear when, exactly, the soldiers were killed.
In a video released by the hardline group and posted to YouTube by the Observatory, the three men are seated before a black flag bearing the Muslim profession of faith and surrounded by masked men holding kalashnikovs.
They give their names and hometown while the man interrogating them identifies them as Alawites, a minority offshoot of Shiite Islam to which President Bashar al-Assad and his senior officials belong.
One is accused of raping a young woman.
The video then showed the mangled bodies of the men lying in a ditch.
"The dogs of Assad have violated the women of Deir Ezzor, and this is what will happen to anyone who does such a thing," a man shouted from behind the camera.
Observatory director Rami Abdel Rahman told AFP that the men were captured on Saturday, but no date was given for their executed.
Al-Nusra, which first gained notoriety for its suicide bombings in Syria, has evolved into a formidable fighting force leading attacks on battlefronts throughout the embattled country.
Its extremist tactics and suspected affiliation to the Al-Qaeda offshoot in Iraq have landed it on the US list of terrorist organisations.
The new killings come on the heels of a UN report, which said the situation in Syria was continuing to degenerate amid a "proliferation of serious crimes including war crimes, and most probably crimes against humanity, by both sides."