BEIRUT -- The U.N. envoy for Syria said Thursday a cease-fire in Syria was "largely holding with some exceptions" as opposition activists reported a mounting number of government airstrikes, including a raid in the northern Aleppo province that killed at least six civilians.

Speaking to reporters in Geneva, Staffan de Mistura said he was concerned fighting northwest of Damascus that has cut off the capital's clean-water supply would escalate and derail proposed negotiations between the government and the opposition in Astana, Kazakhstan, later this month.