At first glance, the training camp appears no different from the many others shown in propaganda videos posted by al-Qaida's affiliate in Syria. Hooded recruits in camouflage shoot at targets or march in formation under the black flag of the Islamic State of Iraq and Syria (ISIS).

But look closer and the "fighters" appear quite small. The tallest are barely chest-high to their instructors, and the shorter ones wear ill-fitting uniforms and appear to struggle under the weight of their weapons. A photo of the recruits without their hoods confirms all of them are young boys.

They are "Zarqawi's Cubs," the youth brigade of Syria's most fearsome Islamist rebel group and one of the newest manifestations of al-Qaida's deepening roots in rebel-controlled sections of the country. Building on earlier efforts to expand their influence in Syrian schools, radical Islamists appear to be stepping up efforts to indoctrinate and train children, some as young as 10, according to independent experts who have studied the phenomenon.