Rebels attempt to break regime control of roads leading into Aleppo amid fears that supplies could run out for its 300,000 residents in three weeks.
Syrian opposition forces are pushing into regime-held areas of Aleppo in an effort to break the siege on the country's largest city.
The rebels said they took positions in the city's southwest, just one week after a regime offensive drove them out of northern Aleppo.
Regime sources confirmed the rebel coalition, which consists of Ahrar al Sham and former Al Qaeda affiliate Jabhat Fatah al Sham, had launched an offensive on an air force artillery base, but were forced to retreat when regime forces fought back.
The rebel fighters have been on the back foot since regime forces captured the strategic Castello road which connects Aleppo to neighbouring Turkey last week, thus blocking off a vital supply route for the rebels.
Regime forces have amassed at the southern part of the road as they prepare to make inroads into rebel held districts.
Fears have been growing for some 300,000 civilians still trapped in the city, with a new wave of massive inner-city operations likely to be launched soon.
Aleppo has been divided between regime and rebel forces since 2012, forcing the majority of its population to flee their homes.
Rebels had the upper-hand until Russia decided to back the embattled regime with air strikes last year.
On Thursday, Syrian regime and Russian planes dropped leaflets around the city telling rebel fighters to surrender and civilians to leave. They have designated routes for civilians to escape the siege, but by Saturday only 169 people had left.
But the UN has condemned the plan, warning that supplies in the city could run out within three weeks, while rights groups have said the regime is giving civilians a choice between displacement and starvation.
The US has claimed the move may in fact be an attempt to depopulate the city so the regime can take control.
Over five years of war in Syria has failed to break the deadlock between the regime and rebels groups, despite numerous attempts to implement ceasefires and establish dialogue.
According to some estimates, as many as 470,000 people may have died in the war.
As much as half of Syria's populations has been displaced, triggering the worst refugee crisis the world has seen since World War II.