Turkey is negotiating with Russia to open a corridor to evacuate Syrian rebel fighters and civilians from the remaining opposition-held districts of Aleppo, but no agreement has yet been reached, a senior Turkish official told Reuters.

Rebel defenses in Aleppo collapsed on Monday, leading to a broad advance by the Syrian army across more than half of the remaining insurgent pocket in the city and a retreat of opposition fighters to a few districts.

"Efforts are still underway to open a corridor for the rebels and civilians in Aleppo, and evacuate them from the region. There has been no agreement on this issue yet," the Turkish official said, speaking on condition of anonymity.

"Turkish and Russian officials are expected to meet in Turkey tomorrow and evaluate the situation. The meeting will put the issue of opening a corridor, as well as a ceasefire, on the table," he said.

The official also said Turkey was continuing its efforts with the United States, as well as Iran, the European Union and Gulf countries to help evacuate opposition fighters from Aleppo.