Fighters from Islamist Syrian rebel group Jabhat al-Nusra take their positions on the front line during a clash with Syrian forces loyal to President Bashar al Assad in Aleppo in this December 24, 2012 file photo. The most feared and effective rebel group battling Syrian President Bashar al-Assad, the Islamist Nusra Front, is being eclipsed by a yet more radical jihadi force. Al Qaeda's Iraq-based wing, which nurtured Nusra in the early stages of the rebellion against Assad, has moved in and sid