BEIRUT, Dec 18 (Reuters) - The Syrian Observatory for Human Rights war monitor said on Sunday a rebel group formerly known as the Nusra Front was preventing evacuation buses from entering the besieged villages of al-Foua and Kefraya in Idlib province.

An evacuation of rebel-held eastern Aleppo stalled this week because of demands from pro-government forces that a group of people needing medical treatment also be allowed to leave al-Foua and Kefraya, which are besieged by insurgents.

(Reporting by Lisa Barrington; Editing by Keith Weir)