GENEVA - UN Special Envoy for Syria Staffan De Mistura on Thursday called the decision of the United States and Russia to break off cooperation as a "serious setback," adding that "one can deny that we are in an emergency mode" regarding the conflict in Syria.

He told reporters during a press conference that the number of al Nusra fighters in eastern Aleppo area is "at maximum 900 people".

"Are we talking about 900 people being the reason for destruction of the city?" he stressed.

He added that UN estimated that in eastern Aleppo there is around 8,000 opposition fighters, while the entire population there is around 275,000, including 100,000 children.