Freezing temperatures and burning tents have heightened the misery of thousands of migrants pushing travelling through Europe.

A fire broke out at a camp for migrants in Slovenia, the current gateway to Austria and beyond. The cause of the fire at the camp in Brezice, on Slovenia's border with Croatia, is not clear, but migrants had been lighting fires outside their tents to ward off the chilly autumn weather.

Firefighters extinguished the flames which consumed several tents, while women and children were evacuated from the camp. Many of those at the Brezice camp arrived in the dark of night after wading or swimming across the Sutla River in temperatures close to freezing.

Serbia, Croatia and Slovenia have struggled to cope with the migrants travelling through the Balkans, their journey made more difficult since Hungary erected fences protected by razor wire, police and soldiers on its southern borders, forcing migrants to find new routes west.

Hungary closed its border with Serbia to the free flow of migrants on September 15 and clamped down on its border with Croatia on Saturday. Since then, more than 20,000 migrants have entered Slovenia from Croatia, with many thousands more on their way.

Slovenian politicians have granted more powers to the army to work with police in managing the migrant influx along the borders of the small Alpine nation.

Further back, hundreds of migrants pushed into Croatia after spending the night out in the open in freezing cold, waiting to cross from Serbia.

Migrants walked down the muddy border passage and over corn fields. Croatian police had deployed on the boundary to stop them but then moved away.

"I am sorry for Europe," said Iraqi migrant Ari Omar in a field in Rigonce, Slovenia, on the border with Croatia. "We did not think Europe is like this. No respect for refugees, not treating us with dignity. Why is Europe like this?"

UN refugee agency officer Francesca Bonelli said around 3,000 migrants were there overnight, including little children, older people, people in wheelchairs and many who are sick and exhausted.

The European Union's executive, meanwhile, summoned leaders of the countries on the migrant trail to a summit in Brussels on Sunday in an effort to better coordinate the flow from one country to another.