"Serbia risks becoming a dumping zone, a new Calais where people are stranded and stuck," Andrea Contenta, humanitarian affairs officer for Medecins Sans Frontiere (MSF) in Serbia told the Guardian newspaper, referring to the French encampment cleared by authorities last fall.
"They choose to stay here in Belgrade, even if it's terrible," MSF press officer Gemma Gillie added. "They still have their freedom. If they do want to carry on their journey, which a lot of them do, they can. And so they're waiting here."
Some 2,000 people are living out of warehouses and empty train cars in Belgrade with no running water and no sanitation, the organization says. MSF runs a mobile clinic, treating about 100 people daily with coughs, colds, hypothermia and frostbite.
The UN estimates about half of the people currently taking shelter in Belgrade are younger than 18 years old, and MSF says they see patients as young as 7 or 8 years old traveling with older siblings.
"The mothers I have met arriving here are distressed because they are unable to keep their babies warm and safe. We see children with early signs of hypothermia such as blue lips and hands, as well as high fevers and respiratory problems," she said. "Instead of focusing on closing their borders, Europe’s governments should be doing more to give people fleeing war a dignified and humane reception.”
"I saw one man brought in who was unconscious from the cold," Tahir Bakhtiary, an MSF translator told the Sunday Times. "Something has to be done. There is nothing there. It could get a lot worse. People could die."
And they are dying. Aid agencies say at least five asylum-seekers have died in Europe, and more could follow when temperatures plummet this week.
"Saving lives is the most urgent priority right now," UNHCR spokeswoman Cécile Pouilly told reporters. "We are extremely worried about continued reports of push-backs in all countries along the Western Balkans. These practices are simply unacceptable and must be halted, as they place the lives of refugees and migrants at heightened risk and violate their most fundamental rights."
Speaking to Save the Children in Serbia, a man using the pseudonym Nasir, who fled the war in Syria five months ago with his wife and two small children, said they never expected this.
"The boat journey was the hardest part. It was extremely cold, everything was wet and the babies were ill," he said. "Sometimes I fear for my children. We couldn’t remain in Syria, but it doesn't get this cold there. We have never been this cold."
English language graffiti pleading for help is scrawled across the walls in Belgrade.
"No one leaves home unless home is the mouth of a shark," reads one message, in an image snapped by the MSF press officer, Gemma Gillie. Other messages read: "We are helpless" and "Please don't forget about us."
Of the more than 7,000 refugees, asylum-seekers and migrants currently in Serbia, most are in heated government shelters. But it's the nearly 2,000 that remain outside, unregistered, that has aid agencies terrified.