Leading children's charities warned Tuesday that young refugees are at serious risk from the bitterly cold Balkan weather, as figures showed 31,000 migrants have arrived in Greece already this year.

The number landing on Greek shores so far this month is 21 times higher than for all of January 2015, the International Organisation for Migration said, despite a three-billion-euro ($3.3 bn) deal between the European Union and Turkey that aimed to stem the flow.

As refugees continued to flow from Greece through the Balkans on their way to western Europe, aid workers sounded alarms over inadequate shelter from the current freezing temperatures and snowy conditions, particularly for children.

"It's an absolutely desperate situation," Valentina Bollenback, a spokeswoman for Save the Children, told AFP by telephone from southern Serbia near the Macedonian border, where the ground is currently covered with about six inches (15 centimetres) of snow.

She said refugees were forced to trudge for two kilometres (about 1 mile) to cross the border into Serbia. They then travel to the Presevo registration centre, where, despite heated tents, she had seen shivering children with chattering teeth and blue lips.

"There's an increasing risk of hypothermia, pneumonia, and other life-threatening illnesses," Bollenback told AFP.

UNICEF, the UN Children's Fund, also released a statement warning that children arriving in southeast Europe were "physically exhausted, scared, distressed and often in need of medical assistance".

"The recent sub-zero temperatures and sometimes snowy conditions (are) exacerbating the children's poor physical condition as many children on the move do not have adequate clothing, or access to age-appropriate nutrition," the statement added.

"This has been worsened by the lack of shelter and inadequate heating in some reception centres as well as buses and trains."

Nearly 7,000 refugees entered the Presevo reception centre from Friday to Monday, according to Mirjana Milenkovski, a spokeswoman for the UN refugee agency in Serbia, who said the situation was "under control".

But Austria's plans to take a tougher line on migrants, with its army set to start thorough identity checks Wednesday and a mesh fence nearly completed at its main border crossing with Slovenia, could cause a bottleneck and tighter controls further down the migrant trail.

Austria has also signalled that it would follow neighbouring Germany's lead and begin turning back any new arrivals seeking to claim asylum in Scandinavia.

- 'Catastrophic failure' -

More than one million refugees and migrants crossed the Mediterranean Sea to Europe in 2015, nearly half of them Syrians.

Relief agency Doctors Without Borders (MSF) on Tuesday denounced the European Union's "catastrophic failure" to help the waves of asylum seekers, demanding that safe transit corridors be provided for them.

The International Monetary Fund also warned Europe of the economic challenge of the refugee crisis and urged it to work harder to assimilate migrants.

"The tide of refugees is presenting major challenges to the absorptive capacity of European Union labour markets and testing political systems," the IMF said in its latest global economic outlook.

"Policy actions to support the integration of migrants into the labour force are critical to allay concerns about social exclusion and long-term fiscal costs."

Greece's shipping minister has meanwhile criticised coastguards who forced an alleged Turkish smuggler to look at the bodies of three dead migrant children who died crossing the Aegean until he broke down in tears.

"Clearly this is exaggerated behaviour," Shipping Minister Thodoris Dritsas said late Monday in a statement regarding the incident, which was shown in footage broadcast on Sky News.

"The state and its officials should be cool-headed and professional against any detainee, even one accused of heinous crimes," Dritsas said.

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