The Syrian Observatory for Human Rights says Syrian rebels fighting government forces have linked up with fighters from besieged eastern Aleppo, breaking a month-long siege.
But it said no secure corridor had yet been established between the two rebel-held territories.
The Observatory and a witness said heavy fighting and air strikes were continuing in the Ramousah area of southwestern Aleppo, home to a major government military base, where the siege was breached after a rebel offensive that began this week.
Taking control of the Ramousah complex, which contains a number of military colleges, would isolate government-held western Aleppo by cutting the southern route out towards the capital Damascus.
It would also give rebels access to armaments stored in the base, which has been used by the Syrian army in the country's five-year conflict as a centre from which to shell opposition targets.
Jabhat Fatah al-Sham, formerly the al Qaeda-affiliated Nusra Front, said in an online statement: "Fighters from outside the city met their brother fighters from inside the city, and work is under way to establish control over remaining positions to break the siege."
A commander from another, more moderate rebel group also told Reuters the siege had been broken but said it was early days and matters were "not easy".
The Observatory, which monitors the fighting, said rebels on Friday took control of the Weaponry College and part of the Artillery College. But the Syrian army said it had repelled the attack.
A state television reporter said the Syrian army had closed the Ramousah road to protect civilians from a rebel advance, and a large number of army reinforcements had arrived.
Syrian President Bashar al-Assad wants to take full control of Aleppo, pre-war Syria's most populous city, which has been divided between rebel and government-held areas. Such a victory would be a crushing blow to the insurgents.
A quarter of a million civilians are thought to still live in Aleppo's opposition-controlled eastern neighbourhoods, effectively under siege since the army and allied militia cut off the last road into rebel districts in early July.