Syrian opposition forces have taken control of much of the country’s second largest city Aleppo, according to video verified by CNN and residents in the city. A rebel alliance launched a surprise attack this week,

Rebel forces opposing President Bashar al-Assad have launched their biggest offensive in years this week, controlling a majority of Syria's second city of Aleppo according to a monitor.

The rapid advance on Aleppo came just four days into a surprise rebel offensive that is the most intense escalation in years in a civil war that was mostly dormant.