The battle for Syria's Aleppo, which is key for both the regime and rebels and the most important so far in a five-year civil war, is likely to be protracted.
Both sides have massed troops around the country's northern second city, which has roughly been divided since 2012 between a regime-held west and a rebel-controlled east.
How strong are the rebels?
The Army of Conquest - which in 2015 drove the regime out of most of the northwestern province of Idlib - is the main rebel coalition in this battle against forces loyal to President Bashar al-Assad.
How strong are the rebels?
The Army of Conquest - which in 2015 drove the regime out of most of the northwestern province of Idlib - is the main rebel coalition in this battle against forces loyal to President Bashar al-Assad.
The alliance includes a dozen groups including the Fateh al-Sham Front - which used to have ties with Al-Qaeda - and the Islamist faction Ahrar al-Sham.
According to military experts, the alliance consists of 30,000 to 40,000 well-trained and motivated fighters including around 10,000 in Aleppo province. Among them are several thousand foreign jihadists.
They have tanks, armored personnel carriers and artillery - mostly captured from the Syrian army.
They also have US-made anti-tank TOW missiles. READ MORE