Up to 16,000 civilians have fled eastern Aleppo as the rebels lost all of the northern neighbourhoods of their stronghold, the UN said Tuesday, describing the situation as "chilling".

"The intensity of attacks on eastern Aleppo neighbourhoods over the past few days has forced thousands of civilians to flee to other parts of the city," UN humanitarian chief Stephen O'Brien said in a statement, adding that reports from the UN's humanitarian partners indicated that "up to 16,000 people have been displaced, many into uncertain and precarious situations".