The U.N. envoy for Syria says this is the most "violent and worrying and dangerous" moment in the country in four years, pointing to escalating violence on multiple fronts that undermines prospects for peace and regional stability.

Staffan de Mistura reiterated Secretary-General Antonio Guterres' appeal to those fighting and all concerned "to de-escalate immediately and unconditionally." And he urged Russia, Iran and Turkey especially "to use their influence to help reduce violence."

He spoke at a Security Council meeting Wednesday and was followed by U.S. Ambassador Nikki Haley and Russian Ambassador Vassily Nebenzia, who traded accusations about responsibility for blocking the path to peace.

Haley demanded Russia use its leverage to ensure a road to peace. Nebenzia asked the U.S.-led coalition to prevent opposition "provocations."