By Mariam Karouny BEIRUT, March 5 (Reuters) - Al Qaeda's official Syrian wing, the Nusra Front, announced on Thursday the death of its top military commander, who insurgent sources said fell victim to a blast targeting a high-level militant meeting. General Military Commander Abu Humam al-Shami, a veteran of Islamist militant fighting in Afghanistan, Iraq and Syria, was the senior-most member of the group to die in the Syria war, an insurgent source said.