A video released online by Islamic State on Wednesday purportedly showed the beheading of a man the group said was a Russian spy its fighters had captured and who had been in Syria and Iraq since last year, the SITE monitoring group reported.

The video shows the man sitting in an orange jumpsuit and giving details of his apparent recruitment by Russian intelligence services.

Then, in a different outdoor location, an Islamic State fighter, who in Russian threatens Russia and President Vladimir Putin with attacks, appears to cut the man's throat and cut his head off.

It appeared to be the first claim by the group that it was holding the Russian prisoner. Moscow has not reported that any of its citizens are held by Islamic State.

Reuters could not immediately verify the authenticity of the video.

SITE said the video was from Raqqa province, Islamic State's main stronghold in northern Syria.

Russia's security service, the FSB, declined to comment. There was no immediate comment from Russia's foreign ministry.

Islamic State has beheaded an unknown number of hostages in numerous videos, both Western prisoners and people from the Middle East, including soldiers fighting against it and Muslims who reject its hardline version of Islam.