A video has emerged purporting to show Russian mercenaries torturing and beheading a Syrian army deserter, in what is thought to be rare footage of Moscow's shadowy paramilitary outfit operating in Syria.

The video, which dates to 2017 but was only released in full on Russian social media site VK last week, appears to show Russian-speaking fighters striking the man with a sledgehammer before decapitating him and lynching his body.

It came to the attention of online sleuths who used open-source intelligence to geolocate the footage to al-Shaer oil fields in Homs, central Syria, which was under Russian and Syrian government control at the time of the attack.

Russia has been fighting on the side of Syrian President Bashar al-Assad since it intervened in the war in 2015.

The Syrian army, depleted by mass defections and casualties, had been struggling before Vladimir Putin, the Russian president, decided to support the regime with air cover and troops on the ground.

The video is understood to have been posted last week on a private VK account linked to Russian security contractors called Wagner Group.