At least 25 people were killed in northwestern Syria in air strikes carried out by the Syrian government and Russia, the Syrian opposition-run rescue service known as the White Helmets said early on Monday. 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. Residents said one attack hit a crowded residential area in the centre of Idlib, the largest city in a rebel enclave near the Turkish border where around four million people live in makeshift tents and dwellings.