Channel 4 News: Twelve-year-old Mohamed Asaf’s days are filled treating Aleppo’s war wounded. He starts work at 8am and usually gets to bed by 11pm.

When filmmaker Marcel Mettelsiefen meets him in the city’s Dar al-Shifa clinic, Mohamed is battling to save a young girl’s life.

After months bearing witness to the human tragedy of Syria’s civil war, he has become desensitised to the horrors: “With time it has become easy: blood has become like water to me,” he says.

In an interview, Mettelsiefen describes the logic of the Assad regime which targets civilians while avoiding striking the al-Nusra Front’s command in Aleppo. The desired effect is to radicalize the population, swell al-Nusra’s ranks and thereby heighten Western fears about the fall of the regime. In effect, al-Nusra has become Assad’s insurance against intervention.