The Syrian Observatory for Human Rights on Saturday said the execution took place at the ancient Romanian Theatre in the millennia-old oasis city of Palmyra, which was overrun by the IS militants last May.
The video showed a long line of soldiers on their knees while masked IS militants standing behind them and pointing their guns against their heads at the theatre with a giant IS flag hung behind.
The Observatory, which relies on a network of activists on ground, said the execution probably took place around May 27.
The Britain-based watchdog group said the IS militants who carried out the execution were from the terror group’s young fighters.
Other reports said the execution was implemented by the so-called Caliphate Cubs, a group of young children brainwashed by the IS.
After storming Palmyra, the IS militants blew up the city’s military prison and several Islamic tombs.
On Friday, fresh reports emerged from the city that the IS blew up a big statue of a lion.
Palmyra contains the monumental ruins of a great city that was one of the most important cultural centres of the ancient world.
From the 1st to the 2nd century, the art and architecture of Palmyra, standing at the crossroads of several civilisations, married Graeco-Roman techniques with local traditions and Persian influences, according to the UNESCO.
Syria has many prehistoric, Greek, Byzantine and Islamic heritages. Before the crisis, Syria had attracted many multinational archaeological missions coming for searching new clues of historical facts on the development of civilizations.
The Unesco has listed six Syrian sites on the World Heritage List, including the old cities of Damascus and Aleppo, al-Madhiq castle, the Krak des Chevaliers, the ancient city of Bosra and Palmyra and the ancient villages in northern Syria.