By Suleiman Al-Khalidi, Sylvia Westall and Tom Perry AMMAN/BEIRUT (Reuters) - Syrian insurgents say they have begun a campaign to capture full control of the divided city of Aleppo, Syria's most populous city before it become a main battleground in its four-year-old civil war. Neither side has been able to control Syria's main commercial hub, 50 km from Turkey, since battle erupted there in 2012, turning its UNESCO-listed historic center into a ruin. The prospect of a rebel offensive in the city is a stark sign of the turn of momentum against President Bashar al-Assad's government, which has lost swathes of territory to fighters in recent months in the **rthwest, east and south of the country.