"While strongly condemning this very grave attack on civilians by armed opposition forces, the Special Envoy reaffirms that this should not justify in any case a retaliation on populated areas through barrel bombs by the Syrian government, which is expected like any government to refrain from killing its own civilians," de Mistura’s office said in a statement.
The envoy also called both sides to the Syrian conflict to always respect international humanitarian law.
Syria has been in a state of civil war since 2011, when a series of anti-government protests turned into an armed conflict, with government forces fighting a number of rebel groups.
Aleppo, Syria's largest city, has been the scene of intense fighting for over three years, as armed militant groups try to gain control of the northern part of the country.
Four years of civil war in Syria have led to over 220,000 deaths, according to UN estimates.
