More than 10,000 refugees and migrants are currently stranded in Serbia because of restrictions imposed by countries further away in western Europe, according to the UN refugee agency.
"We can only say that there are more than 10,000 refugees in Serbia," UNHCR spokeswoman Melita Sunjic was quoted by Reuters as saying from the Serbian-Croatian border.
"It is like a big river of people, and if you stop the flow, you will have floods somewhere. That's what's happening now."
Sunjic said there was a shortage of food and blankets. "We are missing everything," she said.
Refugees travelling through the Balkans risk freezing to death as winter begins, the president of the European Commission has warned.
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Ministry of Defence says HMS Richmond has arrived to join patrols off the coast of Libya aimed at stopping human traffickers.