Geneva (AFP) - The United Nations warned Friday that tightening restrictions imposed by countries along the migrant route through the Balkans is creating an increasingly untenable situation, stranding many outdoors amid plunging temperatures.

Hundreds of thousands of migrants -- most of them refugees fleeing conflict and violence in places like Syria, Afghanistan and Iraq -- have crossed the Mediterranean into Europe this year and travelled up from Greece through the Balkans, aiming to start new lives in more prosperous northern European countries.

Alarmed by the influx, officials in Balkan countries at the heart of the migrant route said this week they were restricting the crossings of economic migrants and allowing in only those fleeing conflict.

In a joint statement Friday, the UN refugee and children's agencies, along with the International Organization for Migration, slammed the restrictions, including "people being profiled on the basis of nationality."

"This is becoming increasingly untenable..., not least in light of falling temperatures and the risk for children and others with specific needs," they said.

- 'Serious, serious worry' -

UNHCR spokesman Adrian Edwards pointed out to reporters that the restrictions were "creating tension at border crossing as a domino effect, leaving in total limbo some refugees and migrants stranded at different border points."

"And that, as we head into winter is a serious, serious worry," he said.

He added that the UN and others were providing blankets, winter clothing and other items at key points, but were concerned that the lacking coordination between the countries imposing the restrictions would "lead to a fragmentation of the routes and additional challenges."

IOM spokesman Joel Millman told reporters there were concerns that the numerous restrictions would push migrants to travel through Bulgaria and Albania where conditions are "much more onerous."

UNICEF spokeswoman Sarah Crowe meanwhile stressed that children, who account for a growing percentage of the migrants, are particularly at risk from the plunging temperatures.

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"Especially small babies, children with disabilities all are under heightened risk," she told reporters, adding that the "very large numbers of unaccompanied children" arriving in some countries were vulnerable.

It remains unclear how many children are on the move, but some 214,000 children are currently seeking asylum in Europe, according to UNICEF.

And the numbers of arrivals in Europe do not look set to subside.

Edwards said some 4,000 people are still arriving daily on the Greek islands.

In the past the number of Mediterranean migrant crossings have fallen off in winter, but Edwards pointed out that most of those crossing now are fleeing conflict.

"Refugees don't flee because of the weather and because of the convenience of good weather," he said.

"They flee because they have no choice. It's a decision made in urgency, out of desperation," he said, cautioning: "We have to plan for there being continued arrivals through the winter and through next year."

Meanwhile in Brussels, EU nations agreed Friday to immediately tighten checks on all travellers, including European nationals, at the external borders of the passport-free Schengen area following last week's deadly attacks in Paris, European sources said.