Police have found a group of 31 migrants, 30 of them Syrian refugees, hidden inside a refrigerated truck outside the northern French town of Dunkirk.

The prefecture of the Nord region said they were discovered at a petrol station in the town of Grande Synthe.

All, including a three-year-old child, were in good health despite the truck's low temperature. The non-Syrian in the group was Vietnamese. They were all released.

Many migrants use France as a transit zone on their way to Britain, with thousands who hope to sneak to Britain staying in a squalid migrant camp in Calais, near Dunkirk.

Meanwhile, the Red Cross says vulnerable migrants in the Balkans are facing increasingly difficult conditions with the arrival of heavy rains and cold weather.

It said the deteriorating weather brings with it "severe health and safety risks to people who have already travelled thousands of miles, often with limited access to food, basic necessities, shelter or health care".

Serbia is in the centre of the Balkan corridor for migrants who cross from Greece to Macedonia and then via Serbia to Croatia or Hungary.

Some 250,000 migrants are estimated to have passed through Serbia since the beginning of this year.

Vesna Milenovic, an official for the International Federation of Red Cross and Red Crescent Societies, said: "Our main concern is what will happen in the next few weeks and months. Autumn and winter weather can be severe in the region."