Jihadists and their Turkish-backed allies shelled Syria's second city Aleppo on Friday, in a major offensive against government troops that has sparked some of the deadliest fighting the country has seen in years.

Four civilians including two students were killed on Friday in the Syrian city of Aleppo in insurgent shelling of university student dormitories, the state news agency SANA reported.

Syrian rebels have launched a large-scale attack on regime forces in western Aleppo, according to a Free Syrian Army source and local residents, marking the first major flare-up in years between both sides.