Under assault from hundreds of Russian airstrikes over the past week, Aleppo increasingly is falling out of rebel hands. As many as 40,000 Syrians fled the city toward Turkey’s border over the past two days, walking up to 60 miles to get to safety. Turkish Prime Minister Ahmet Davutoglu said Friday that 15,000 people had already arrived at Turkey’s border from Aleppo.

The Russian onslaught on Syria’s keystone city has left many wondering just how long rebels can hold on. During Syria’s five-year civil war, which has killed at 250,000, it’s been surmised that whoever controls Aleppo will prevail.

“The full encirclement of Aleppo City would fuel a humanitarian catastrophe, shatter opposition morale, fundamentally challenge Turkish strategic ambitions, and deny the opposition its most valuable bargaining chip before the international community,” the Washington based Institute of the Study of War concluded on Friday.