Syrian rebels claim to have taken full control of a Scientific Research centre in Aleppo.
Amateur video shows fierce clashes with government forces, described as the most intense insurgent offensive in the city in three years.
Several rebel groups including ISIL militants are fighting forces loyal to President Bashar al-Assad for control of Aleppo, which has been partitioned since 2012.
Government troops say they’ve carried out heavy air strikes on rebel positions on Friday in retaliation.
The renewed fighting led neighbour Turkey to step up security along its border with Syria.
A military convoy of high ranking soldiers were deployed to the city of Gaziantep.
Aleppo lies just 50 kilometres from the Turkish border.
An insurgent alliance including the al Qaeda-linked Nusra Front and the hardline Islamist Ahrar al-Sham said they had set up a joint operations room to run the offensive to “liberate” Aleppo.
Separately, an explosion at a mosque in Syria’s Idlib province on Friday killed at least 25 members of the Nusra Front, according to the British-based Syrian Observatory for Human Rights.