Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the regime control of the country’s second largest city, Aleppo.

At least 372 people have been killed since rebels launched a surprise offensive on Wednesday, according to observers.

Syrian rebel fighters are advancing southward after capturing much of Aleppo, Syria’s largest city, posing the most serious challenge to President Bashar al-Assad in years amid the country’s civil war,