Syrian government forces Sunday gained full control of the Castello Road, the last paved road into rebel-held eastern Aleppo, a monitoring group said.
The government advance means an estimated 200,000 to 300,000 residents can only escape the fighting in the northern city via dangerous dirt roads, the Syrian Observatory for Human Rights said.
Analysts say the government may seek to impose a lengthy siege in order to bring all of Aleppo, Syria's largest city before the war, under its control with minimal casualties among its own troops.
An eyewitness in Aleppo, who asked to be named only as Aref, confirmed that the Castello Road was under the control of pro-government fighters.
The road was already effectively out of use since government forces advanced to firing positions overlooking it ten days earlier.
Aleppo has been divided between a government-controlled western sector and the opposition-held east since a rebel offensive in mid-2012.
Even before the Castello Road was closed, conditions in the eastern sector were dire, with massive damage from constant government air raids and shelling. Medical facilities have repeatedly been hit.