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

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

With Russia and Iran devoting much of their resources to their own wars, the Syrian rebels took Aleppo with little resistance from the Assad regime.