Aleppo defeat not the end for rebels
jamuna news desk: The fall of rebel-held eastern Aleppo would not mean the end of the fight against President Bashar al-Assad.
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.
Mr Sabra, chief negotiator for the High Negotiations Committee (HNC), told the BBC World Service that the loss of Aleppo would "not be the end of the revolution".
"Aleppo is an important place for the revolution but it's not the last place," he said.
"Right now, we have so many places under the power of (the) Free Syrian Army."
He warned that the military campaign being waged by the Syrian government and its allies was "killing a part of the political process".
"Nobody can think about peaceful solutions in these circumstances," he added.
The advances by pro-government forces followed two weeks of relentless aerial bombardment and captured all northern districts held by the rebels in eastern Aleppo.