Cold weather, disease and a lack of shelter and medicine threaten hundreds of thousands of civilians as they flee fighting in Idlib province, in one of the biggest upheavals of Syria's nine-year civil war, aid groups and doctors said.

The migrants, their numbers swelling by the day, are trapped between advancing Syrian government forces, keen to crush the last significant opposition stronghold, and Turkey's closed border.

Some are having to flee by foot, while many others are having to sleep in their cars, as Syrian and Russian warplanes bombard the highways leading north toward Turkey.