US Secretary of State John Kerry has said the Syrian conflict is "in many ways out of control", as he made a fresh push to salvage a fragile truce.
After talks with UN and Arab diplomats in Geneva, he said progress had been made on a plan to reduce violence in the second city of Aleppo.
However, he said more work was needed and there was no guarantee of success.
A cessation of hostilities has reduced fighting in Syria's five-year civil war but has unravelled in recent days.
About 250 people have reportedly been killed in Aleppo in the past nine days.