The UN envoy for Syria today said, that a cease-fire 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. <br/><br/>Speaking to reporters in Geneva, Staffan de Mistura said, he was concerned that fighting northwest of Damascus has cut off the capital's clean water supply and would further escalate and derail proposed negotiations between the government and the opposition in Astana, Kazakhstan, later this month.<br/>'<br/>Rebels say the government's continued campaign for the Barada Valley, the capital's main source of water, has cast the talks in doubt.<br/>