Syria's main opposition and rebel factions in the northern city of Aleppo have rejected a proposal by the U.N. envoy to freeze fighting in parts of the city.

Despite the rejection, U.N. spokesman Stephane Dujarric said on Monday the U.N. will continue to pursue a peaceful solution for Syria, where a four-year conflict has killed more than 220,000 people.

The U.N. envoy, Staffan de Mistura, left Syria after talks with officials to try to arrange a "freeze" in fighting in Aleppo, which has been divided into government- and rebel-held districts since mid-2012.

The opposition said in a statement that it refuses to meet de Mistura if the talks are not based on the premise that a comprehensive solution to Syria's crisis includes President Bashar Assad's exit from office.