Reports say bombing has intensified as more people flee the troubled area.
De Mistura told the European Parliament, referring to the rebel-held districts that Syrian government forces and their allies are storming.
“Clearly, I cannot deny this is a military acceleration and I cannot tell you how long eastern Aleppo will last.
“There is a constant increase of movement on the military side,’’ he said.
Reports say bombing has intensified as more people flee the troubled area. There are reports of civilian causalities including women and children.