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.

NBC News’ Matt Bradley reports on the intensifying violence in the Syrian rebel offensive after Russian and Syrian jets struck a rebel held city in the northern part of the country. Syrian President Bashar Assad is vowed to crush insurgents waging a shock offensive against his regime.