The Syrian opposition stronghold of Idlib, where hundreds of people from eastern Aleppo were being evacuated, is now at risk of a similar escalation of violence, UN envoy Staffan de Mistura warned on Thursday.

"If there is no political agreement and a ceasefire, Idlib will become the next Aleppo," De Mistura said, adding that it was his priority to secure the deployment of UN staff to supervise the Aleppo evacuation.