Syrian air raids on two northern towns held by the militant Islamic State group have killed more than 50 people in two days, activists say.

Islamic State (IS) fighters backed by tanks have captured 16 Kurdish villages over the past 24 hours in northern Syria near the Turkish border, prompting civilians to flee their homes amid fears of retribution by the extremists sweeping through the area, activists said.

At least 10 people have been killed after a missile struck a bus in a rebel-held area of the northern Syrian city of Aleppo, according to activists.

Two government airstrikes have killed at least 29 people in northern Syria, according to activists.

Western powers are preparing a tough response in case Syrian president Bashar al-Assad’s regime deploys chemical or biological weapons, France’s foreign minister said.

Government warplanes bombed a town in northern Syria today, killing at least 18 people, activists said, while the new UN envoy to the country acknowledged that brokering an end to the nation’s civil war will be a “very, very difficult” task.

Humanitarian conditions have grown even more dire in the besieged Syrian city of Aleppo with activists reporting dwindling stocks of food and cooking gas and only intermittent electricity supplies as droves of local residents flee almost two weeks of intense clashes between rebels and regime forces.