"There is not great confidence that this truce can last longer than the previous one," George Sabra told Reuters in a telephone interview. He also said the Syrian government's insistence on controlling aid was obstructing its delivery to Aleppo under the agreement.
Sabra also said it was too early to talk about any resumption of peace talks, and that this hinged on the implementation of humanitarian clauses of a UN resolution passed last year.
In turn, Kremlin spokesman Dmitry Peskov told a conference call with journalists on Wednesday that Syria's moderate opposition must be separated from "terrorist groups" to let the fragile ceasefire advance further.
The Syria ceasefire is raising hope that a peaceful solution to the conflict can be found, he added.