"The trilateral meeting that as you know was a possibility is being postponed, it is not taking place on Monday. It will be a bilateral. But the trilateral is not off the table, just being postponed," Staffan de Mistura told reporters, Reuters reported.

De Mistura, asked about the US administration intent to participate, replied, "There is a clearly an intention to maintain and resume these trilateral discussions, the date and circumstances were not conducive for this to happen on Monday."

All sides have flagged their readiness to allow aid convoys to reach Douma and later other towns in the eastern Ghouta province near Damascus, UN humanitarian adviser Jan Egeland said. "It is very, very urgent that we get to eastern Ghouta."