The boys are seen shooting dead five men identified as captured Kurdish fighters who are kneeling.
A caption claims the white boy in the video is Abu Abdullah al-Britani, who is allegedly British.
All the boys are pictured in full military fatigues and a black hat alongside the four other boys.
Five hostages are shown dressed in familiar orange jumpsuits seen in previous IS propaganda videos, ‘The Times’ reports.
“No one can save the Kurds even with the support of America, France, Britain, Germany, the devils in hell,” the British boy is heard to say in Arabic.
He then calls for the ‘takbir’ -- the term referring to the phrase ‘Allahu akbar’ -- which the other boys shout in unison, raising their pistols to the sky before shooting the hostages dead.
The video then pans to gunshot wounds to the men’s heads as they lie dead in the sand.
The four other boys in the footage are captioned as being Tunisian, Kurdish, Egyptian and Uzbekistani in origin.
There is no confirmation of the identity of the British boy from the UK Foreign Office but it is believed he could be the son of a white-British woman convert and was taken to join IS at the age of 10.
The name Abu Abdullah al-Britani may refer to his adopted father -- a young British terrorist who married in Syria and was killed in a drone strike.
Children have featured prominently in IS propaganda in which the militant group boasts about preparing the next generation of fighters.
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