A video distributed Thursday by the Syrian Observatory for Human Rights shows a jihadist in the east of the country executing supporters of the regime of President Bashar Al-Assad.
The Britain-based watchdog did not specify whether the nine men killed were combatants, nor did it say when the footage was recorded.
Later Thursday, the Britain-based group sent further footage showing what it described as the body of the reported executioner -- a man of Saudi origin identified as Kasura al-Jazrawi.
He was killed fighting locals in oil-rich Deir Ezzor in eastern Syria over fuel, Observatory director Rami Abdel Rahman told AFP.
Large swathes of Deir Ezzor are out of regime control, and observers have warned of increasing in-fighting among jihadist and local rebel groups in insurgent-held areas.
The men allegedly executed by Jazrawi of Al-Nusra Front, a jihadist organisation operating in Syria that the United States has classed as "terrorist", "are supporters of the Assad regime, but we cannot know whether they were regime troops", Abdel Rahman said.
The online video shows a masked man reading out a statement. At least nine blindfolded men kneel in front of him, facing the camera.
Behind the men are two black flags.
"Al-Nusra Front's sharia (Islamic law) court in the eastern area of Deir Ezzor (near the Iraqi border) has sentenced these apostate soldiers to death, because of their massacres against our people in Syria," said the man later identified as Jazrawi.
He then takes a pistol and shoots the prisoners, one by one.
Jihadist websites also posted the video, and said the footage was filmed in 2012.
In an act that enraged other Islamists battling Assad's regime in Syria, Al-Nusra Front leader Abu Mohammed al-Jolani pledged allegiance last month to Al-Qaeda leader Ayman al-Zawahiri.
Al-Nusra Front is reputed to be among the fiercest and best-equipped rebel groups fighting the regime.
It has said it carried out most of the suicide attacks to have struck Syria during the course of the conflict, which broke out when a peaceful movement for change morphed into a bloody civil war after the regime unleashed a brutal crackdown against dissent.