Under a "humanitarian pause", eight corridors have been opened to allow more than 250,000 civilians and opposition forces in besieged parts of the city to leave.
An eleven-hour "humanitarian pause" announced by Russia to allow civilians and opposition fighters to leave besieged areas of Aleppo has been extended for a further 24 hours, the Russian Defence Ministry announced.
This is the second pause in fighting between the Moscow-backed Syrian regime and opposition forces in Aleppo this year. But the UN says the temporary halt to hostilities is not long enough to provide any relief supplies to the more than 250,000 people.
On Monday, Russia announced an eight-hour pause in air strikes on the city to take effect on Thursday. It was extended by three hours on Wednesday.
Syrian and Russian warplanes stopped launching air strikes on opposition-held districts from 0700 GMT on Tuesday.
The Syrian regime has said it is opening eight corridors to provide safe passage for civilians to leave the city.
Opposition fighters can use two of the corridors but they must leave their weapons behind, the regime has instructed.
Clashes were reported on one of the corridors when artillery exchanges erupted around the crossing point on the front line in the city cent, an AFP photographer said. The regime's SANA news agency blamed the opposition groups.
UN envoy Staffan de Mistura said the truce would be long enough to safely evacuate 200 wounded people from the devastated east of the city.
In September, Russia and the United States pushed the regime and the opposition to abide by a ceasefire in Aleppo, but it proved short-lived.
Syrian regime forces and their Russian allies have since stepped up air strikes to take full control the city.
At least 448 people including 82 children have been killed in recent air strikes on the city.