Thousands of refugees and migrants woke up to a chilly morning and a dense fog in a muddy field on the border between Serbia and Croatia. Many of those ‎camping in the open are families with children, waiting to continue their journey into the European Union.

About 3,500 migrants spent the night in freezing cold at the Berkasovo-Bapska crossing on the Croatian-Serbian border, huddling in tents or under tarpaulin provided by aid groups and burning bonfires to keep warm. Aid agencies are concerned about backlogs of migrants building in the Balkans, battered by autumn winds and rain as temperatures drop before winter. "The keyword is: cold. It's been freezing, this was our busiest night this week," said ‎Astrid Coyne-Jensen of the Danish People's Aid medical team.

A refugee named Mustafa, wrapped in a blanket, said his group of three families spent the night at the crossing under a piece of discarded tarpaulin as there were not enough tents for everyone. "I am from Hama in Syria, I have family in Turkey, I will tell them not to ‎come as this is not for people, this is for animals," he said.

A group of refugees gathered around a camp fire and sung a song about Germany, the ultimate destination for a majority of them. "Germany, Merke‎l, we love you, we love you, Germany, we love you, Merkel," they sang in Arabic, showing a photo of Germany's chancellor Angela Merkel on a mobile phone that was passed along the crowd.

From Serbia, migrants and refugees enter Croatia and from there they generally head for Slovenia after Hungary sealed its borders with Croatia and Serbia. Europe's biggest refugee and migrant crisis since World War II has seen hundreds of thousands of people fleeing war and poverty in the Middle East, Asia and Africa this year, with hundreds losing their lives in risky voyages across the Mediterranean.