Islamic State has recruited at least 400 children in Syria in the past three months and given these so-called "Cubs of the Caliphate" military training and hard-line indoctrination, a monitoring group said Tuesday.

The Syrian Observatory for Human Rights said the children, all aged under 18, were recruited near schools, mosques and in public areas where Islamic State carries out killings and brutal punishments on local people.

One such young boy appeared in a video early this month shooting dead an Israeli Arab accused by Islamic State of being as spy. A French police source said the boy might be the half-brother of Mohamed Merah, who killed three soldiers, a rabbi and three Jewish children in Toulouse in 2012.