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.

Russian and Syrian warplanes targeted insurgents in an Aleppo city suburb on Saturday, two Syrian military sources said, after the rebel fighters penetrated the heart of the city in Syria's northwest in a surprise attack the previous day.