Opposition forces have withdrawn from six neighbourhoods in the city, their last holdouts in Aleppo.
The Syrian regime and its allies have retaken the city of Aleppo, the Britain-based Syrian Observatory for Human Rights said on Monday.
Rami Abdulrahman, the director of the Observatory, said that the battle for Aleppo has reached its end after sweeping advances by the regime forces into the opposition-held sector.
Opposition forces have withdrawn from six neighbourhoods in the city, their last holdouts in Aleppo, he added.
There was no official confirmation from the Syrian regime or its ally, Russia.
Earlier, there were reports that regime leader Bashar al Assad was close to taking back full control of Aleppo, which was Syria's most populous city before the war and would be his greatest prize so far after nearly six years of conflict.
Rebel groups in Aleppo received a US-Russian proposal on Sunday for a withdrawal of fighters and civilians from the city's opposition areas, but Moscow said no agreement had been reached yet in talks in Geneva to end the crisis peacefully .
While Aleppo's fall would deal a stunning blow to rebels trying to remove Assad from power, it would still be far from enough to restore regime control across Syria. Swathes of the country remain in rebel and terrorist hands, and on Sunday Daesh retook Palmyra.
The loss of Palmyra, an ancient desert city whose recapture from Daesh in March was heralded by Damascus and Moscow as vindicating Russia's entry into the war, is an embarrassing setback to Assad.