Syrian militants reach central Aleppo as government forces appear to retreat

Surprise offensive by rebels marks biggest challenge to Bashar al-Assad's control in years

Islamist rebels once exiled to a mountainous pocket of the Syrian countryside now roam the streets of central Aleppo, taking pictures below its ancient citadel and tearing down symbols of President Bashar al-Assad's rule.

The surprise offensive in which insurgents seized territory across north-western Syria appears to have dramatically shifted the balance of power in Aleppo, the country's second largest city, and marks the most serious challenge to Assad's control in years.