Isis social media accounts have published a video which purports to show the beheading of a Russian soldier.
The video is 12 minutes long and shows the beheading of a man reportedly named Pterinko Afghene.
The soldier, wearing a black outfit, is seen kneeling before an Isis militant. Prior to his death, Afghene says "this idiot believed the promises of his state not to abandon him if he was captured" and urges the Russians to cease their attacks on Isis.
The executioner, a bearded man, also speaks Russian.
It has been reported that Afghene was a Russian spy who had infiltrated the ranks of Isis before sending information back to Moscow. His task was to befriend Isis' 'minister of war', the late Omar Al-Shishani, but he was discovered before the mission could be completed.
Russia, which celebrates Victory Day today (May 9) to remember its triumph over the Nazis in World War II, has yet to confirm the veracity or the video.
The country is a major actor in the war in Syria, supporting strongman president Bashar al-Assad against a proliferation of rebel groups, including Isis.