President Bashar al-Assad's forces used air strikes and artillery to bombard insurgents on several fronts in the 19-month-old conflict, which risks dragging in regional powers with no sign of a diplomatic breakthrough.

Turkey - increasingly entangled after having intercepted a Syrian airliner carrying what it said were Russian-made munitions for the Syrian army, infuriating Moscow and Damascus - has led calls for intervention, including no-fly zones enforced by foreign aircraft to stop deadly air raids by Assad's forces.