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.

Migrants Serbia Croatia
People try to keep themselves warm around a fire near the village of Berkasovo, SerbiaMarko Djurica/Reuters
Migrants Serbia Croatia
Migrants and refugees, wrapped in blankets, walk through the fog near the village of Berkasovo, SerbiaMarko Djurica/Reuters
Migrants Serbia Croatia
A woman wrapped in a thermal blanket carries a child as she walks through a field to cross the border with Croatia near the village of Berkasovo, SerbiaMarko Djurica/Reuters

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.

Migrants Serbia Croatia
A child covered in a blanket looks up as people wait to cross the border into Croatia from SerbiaMarko Djurica/Reuters
Migrants Serbia Croatia
A migrant covered in blankets walks past an abandoned building near the border with CroatiaMarko Djurica/Reuters
Migrants Serbia Croatia
Opened cans of fish cans are warmed up on a bonfire near the village of Berkasovo, SerbiaMarko Djurica/Reuters
Migrants Serbia Croatia
A man assists local volunteers to remove rubbish from the makeshift holding area near the village of Berkasovo, SerbiaMarko Djurica/Reuters
Migrants Serbia Croatia
People try to keep themselves warm by a bonfire as they wait to cross the border into CroatiaMarko Djurica/Reuters
Migrants Serbia Croatia
Migrants and refugees wait to cross the border between Serbia and Croatia near the village of BerkasovoJeff J Mitchell/Getty Images
Migrants Serbia Croatia
A man carries his baby wrapped in a thermal blanket after crossing the Macedonian-Serbian borderArmend Nimani/AFP
Migrants Serbia Croatia
People carry their children wrapped in blankets as they walk on a dirt road near the Serbian village of MiratovacArmend Nimani/AFP
Migrants Serbia Croatia
A woman carries a child along a dirt road near the village of Miratovac, SerbiaArmend Nimani/AFP
Migrants Serbia Croatia
People huddle together under blankets to keep warm while waiting to cross into CroatiaJeff J Mitchell/Getty Images
Migrants Serbia Croatia
Migrants and asylum seekers wait for a bus after crossing the Macedonian-Serbian border near the village of MiratovacArmend Nimani/AFP
Migrants Serbia Croatia
A child eating a piece of bread waits for a bus after crossing the Macedonian-Serbian border near the village of MiratovacArmend Nimani/AFP
Migrants Serbia Croatia
A child cries while sitting in the mud in a field near BerkasovoJeff J Mitchell/Getty Images
Migrants Serbia Croatia
Men wrapped in blankets hold children as they wait near the village of Berkasovo in SerbiaJeff J Mitchell/Getty Images
Migrants Serbia Croatia
A child waits in a muddy field strewn with blankets near the border between Serbia and CroatiaJeff J Mitchell/Getty Images
Migrants Serbia Croatia
A woman wrapped in a thermal blanket holds a baby as they wait in a muddy field near the border between Serbia and CroatiaJeff J Mitchell/Getty Images

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.