The militant Islamic State group released a video on Saturday purporting to show the mass execution of 25 Syrian soldiers in a Roman-era amphitheater in the city of Palmyra, AFP reported.

The mass execution allegedly occurred shortly after the city was captured by Islamic State, also known as ISIS or ISIL, on May 21.

The video shows teenaged boys, clad in desert camouflage and brown bandanas, shooting the soldiers dead on the amphitheater's stage in front of a huge ISIS flag.

In the video, only several spectators are seen sitting on the amphitheater's benches – men and some children.

The mass execution was first reported by the Syrian Observatory of Human Rights, a Britain-based monitor, on May 27, only several days after the city fell to ISIS. According to AFP, 200 executions were carried out by ISIS in Palmyra and its surrounding areas when it was captured.

Palmyra is home to one of the most famous world heritage sites in the Middle East, renowned for its Roman-era colonnades and 2,000-year-old ruins. The Islamic State group has proclaimed a caliphate to rule over all Muslims in the territory it holds in Syria and Iraq. ISIS militants have a history of carrying out mass killings in the towns and cities they capture and destroying any ancient monuments they consider evidence of paganism.

However, ISIS has reportedly not damaged the ruins in Palmyra.