The rebels have made stunning gains against President Bashar al-Assad’s forces, with Russia, Iran and Hezbollah distracted by other conflicts.

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.

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