The Islamic State group says it has beheaded a Russian agent who had infiltrated IS ranks as a fighter to spy on the militant group.

A gruesome video released late on Monday purports to show the man, described as a captain in a Russian intelligence department that spies on “terrorists.” His name was only given in Arabic.

The video shows the alleged spy being interrogated by a masked man, at which point he says he was sent to Syria to spy on IS top military commander Omar al-Shishani who was killed last year in a U.S. airstrike, and later, on his knees somewhere in the desert where he is killed by a Russian-speaking man.

Thousands of fighters from the former Soviet Union have joined IS in Syria and Iraq.

(AP)