The UN mediator in the Syrian conflict, Staffan de Mistura, has told the BBC he believes there is a fresh opportunity to resolve the country's crisis.
He said truce measures could succeed because of the common threat from Islamic State (IS) militants, as well as a growing weariness with conflict.
The UN has called for "freeze zones" to halt fighting and improve aid.
More than 200,000 people have died in Syria's increasingly fragmented civil war, now in its fourth year.
Rebel groups such as IS and the al-Qaeda-aligned Nusra Front have been fighting among themselves, as well as against forces loyal to President Bashar al-Assad.