Syria conflict: Aleppo defeat ‘not the end for rebels’
George Sabra added, however, that it would make any hope of a peace deal more difficult.
Syrian forces backed by heavy air strikes have seized more than a third of rebel-held territory in the city.
Rebel factions say they have pulled back to a more defensible front line.
Thousands of civilians have fled besieged districts of Aleppo after heavy fighting over the weekend.
