The execution took place in the north-east Syrian province of Al-Hassakah, according to British-based Syrian Observatory for Human Rights.

It claims the Christian militant took him prisoner when he found out he was a member of ISIS and 'beheaded him in revenge for abuses committed by the group in the religion'.

The Christian was allegedly fighting for Kurdish forces who drove ISIS out of dozens of Assyrian villages in Al-Hassakah earlier this month.

ISIS abducted as many as 220 Assyrian Christians from the north-eastern Province in February and released 19 of them weeks later.

The group kidnapped the hostages after seizing control of their villages from Kurdish control and forced thousands of people from their homes.

The province which borders Turkey and Iraq is predominantly made up of Kurds but a minority of Christian Assyrians and Armenians.

Younan Talia, a senior official with the Assyrian Democratic Organisation, said ISIS raided as many as 33 villages.