Syrian government forces on Sunday gained full control of Castello Road, the last paved road into rebel-held eastern Aleppo, a monitoring group said, amid fears of a humanitarian crisis in the area.
The 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 Aleppo, Syria's largest city before the war, under its control with minimal casualties among its own troops.
The regime's control of the road triggered fighting between government forces and rebels in its vicinity.
Opposition forces were trying to push back regime troops and regain control of the strategic supply route, the observatory said.
No casualties figures were reported.
An eyewitness in Aleppo, who asked to be named only as Aref, confirmed that 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 10 days earlier.
Even before Castello Road was closed, conditions in the eastern sector were dire, with massive damage from constant government airstrikes and shelling. Medical facilities have repeatedly been hit.
Retaliatory attacks on Sunday intensified between government forces and the opposition in Aleppo, according to the Britain-based observatory that relies on a network of activists inside Syria.
Five civilians, including two children, were killed in shelling by rebels on regime-controlled districts in the city.
Unidentified warplanes, meanwhile, bombarded rebel-controlled areas on the western outskirts of Aleppo, the observatory added. No casualties were reported.
Once Syria's commercial hub, Aleppo has been divided between the forces of Syrian President Bashar al-Assad in the west and rebels in the east since fighting erupted for control of the city in mid-2012.