Islamic State (Isis) has released a video purportedly showing the execution of 25 soldiers loyal to Syrian President Bashar al-Assad at the heritage site of Palmyra.

The 10-minute footage, which begins with a recitation of verses from the Koran, shows IS Islamists, mostly teenagers, shooting down the soldiers in an ancient amphitheatre. The soldiers were shot in their heads with pistols.

The video was thought to have been recorded shortly after the extremist organisation took control of the Syrian city on 21 May.

Just before the grisly murder of the Syrian troops, the video shows the aftermath of the fighting in the city. Gruesome scenes of corpses lying in the streets can be seen.

An elderly man, against the backdrop of the notorious IS flag, reads a document to the assembled crowd while IS fighters stand behind the captives, kneeling with their hands tied at the back.

The soldiers, who were captured by IS in Homs, were thought to have been flogged as their backs appear to suggest in the video. The footage could not be independently verified, but it has all the hallmarks of the videos published by the Islamist group.

Dozens of people, mostly residents of Palmyra, are shown watching the slaughter. Some of them are seen waving IS flags while others break down as they watch the execution in horror.

The Syrian Director-General of Antiquities and Museums, Maumoon Abdul Karim, told reporters that IS "barbarically executes people and destroys antiquities in the ancient city of Palmyra".

IS had destroyed the relics with sledgehammers after a Shariah court declared the centuries-old objects anti-Islamic. The group has been destroying several heritage sites in Syria and Iraq in what the Unesco called "cultural cleansing".