Hossam al-Sarmani, a spokesman for the Daewood Brigade of the Free Syrian Army, told CNN the men shown on a video posted on the Internet killing those they call "dogs of Assad" are not part of the FSA.
"This execution did take place, and this is the heinous truth," Sarmani told CNN Friday. "We will find them, capture them and hold them in a prison until they can be tried by international law or the new Syrian law, God willing."
Sarmani called those who killed the captured government soldiers "extremists." He said they appeared suddenly Thursday while FSA forces were engaged in combat, killed the unarmed prisoners and then slipped away.
The video attracted the attention of Amnesty International and other rights groups that say it is another instance of both sides in the Syrian conflict committing atrocities, The New York Times reported Thursday.
In the video, 10 prisoners are shown being forced either to lie next to or on top of each other inside a building.
Their captors, anti-government fighters who cannot be identified, are yelling "Allah Akhbar!" ("God is great!") while kicking the prisoners and corralling them into one area before opening fire.
"This shocking footage depicts a potential war crime in progress, and demonstrates an utter disregard for international humanitarian law by the armed group in question," said Ann Harrison, deputy director of Amnesty International's Middle East and North Africa program. "We will continue to investigate the incident and once again urge all sides to respect the laws of war and to refrain from torturing, ill-treating or killing their prisoners."
The alleged executions occurred in Idlib province.
No group had admitted carrying out the atrocities, the BBC reported, but it has been alleged that al-Nusra Front, an extremist group, was responsible.
The military kept up its airstrikes across Syria, with the Syrian Observatory for Human Rights reporting more than 150 people killed in fighting Thursday.
Amnesty International, based in Britain, said more than 36,000 people have been killed since protests against President Bashar Assad began in March 2011.
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