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.

Russian and Syrian jets struck the rebel-held city of Idlib in northern Syria on Sunday, military sources said, as President Bashar al-Assad vowed to crush insurgents who had swept into the city of Aleppo.

Warplanes targeted the city of Idlib in an attempt to fend off advances made by antigovernment forces that included a stunning assault on Aleppo.