The insurgent offensive, shocking for its speed, has posed the most serious challenge to President Bashar al-Assad in years.

Syrian opposition forces have taken control of much of the country’s second largest city Aleppo after a lightening advance that killed dozens of government soldiers in a major challenge to President Bashar al-Assad.

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.