There are growing fears that Syria's second city, Aleppo, could be taken over by Islamic State (IS) as fighting to the north between government and rebel forces intensifies. With the military being driven out of the adjacent province of Idlib by a newly-formed rebel alliance, the Army of Conquest, focus has shifted to the fate of Aleppo, an ancient trading hub near the border with Turkey that has been a battleground for the past three years.