Refugees and migrants are dying in Europe's cold snap and governments must do more to help them rather than pushing them back from borders and subjecting them to violence, the UN refugee agency said on Friday.

"Children are particularly prone to respiratory illnesses at a time like this. It's about saving lives, not about red tape and keeping to bureaucratic arrangements," Sarah Crowe, a spokeswoman for the UN children's agency UNICEF told a UN briefing in Geneva. "The dire situation right now is Greece."

Cecile Pouilly, spokeswoman for the UN High Commissioner for Refugees (UNHCR), cited five deaths so far from cold and said about 1,000 people including children were in unheated tents and dormitories on the Greek island of Samos, calling for them to be transferred to shelter on the mainland.

Hundreds of others had been moved to better accommodation on the islands of Lesbos and Chios in the past few days.

In Serbia, about 80 per cent of the 7,300 refugees, asylum seekers and migrants are staying in heated government shelters, but 1,200 men were sleeping rough in informal sites in Belgrade.

A woman visits the archaeological site of Olympion Zeus in Athens, after a rare heavy snowfall on the city. (Louisa Gouliamaki/AFP/Getty Images)

The bodies of two Iraqi men and a young Somali woman were found close to the Turkish border in Bulgaria and two Somali teenagers were hospitalised with frostbite after five days in a forest, Pouilly said. The body of a young Pakistani man was found along the same border in late December.

A 20-year-old Afghan man died after crossing the Evros River on the Greece-Turkey land border at night when temperatures were below –10 C. The body of a young Pakistani man was found on the Turkish side of the border with Bulgaria.

"Given the harsh winter conditions, we are particularly concerned by reports that authorities in all countries along the Western Balkans route continue to push back refugees and migrants from inside their territory to neighbouring countries," Pouilly said.

A stranded Syrian refugee boy walks through a show storm at a refugee camp north of Athens. (Yannis Behrakis/Reuters)

Phones confiscated, destroyed

Some refugees and migrants said police subjected them to violence and many said their phones were confiscated or destroyed, preventing them from calling for help, she said.

"Some even reported items of clothing being confiscated thus further exposing them to the harsh winter conditions," she said. "These practices are simply unacceptable and must be stopped."

Joel Millman, spokesman for the International Organization for Migration (IOM), said migrant movements across the Mediterranean had "started out in a big way" in 2017, and the death toll for the year was already 27.

The World Meteorological Organization said a movement of cold Siberian air into southeastern Europe had driven temperatures in Greece, Italy, Turkey and Romania five to 10 degrees lower than normal. Such cold outbreaks happen about once in 35 years on average, the WMO said.