Freezing temperatures across Eastern Europe are imperiling the lives of thousands of refugees, many of whom are sheltered in makeshift tents with no heat, the U.N. said on Friday.

The harsh weather conditions have claimed the lives of five refugees in Europe so far this year, according to the U.N.

"We are deeply worried at the situation of refugees and migrants," Cécile Pouilly, spokesperson for the United Nations High Commission on Refugees, said at a press briefing in Geneva.

"As a life saving measure, we continue to provide heaters, blankets and winter clothes to residents of informal sites who have not yet agreed to move to government centers," Pouilly added.

Some of the most vulnerable groups include about 1,000 people, among them families with young children, on the Greek island of Samos, and 1,200 males -- up to a quarter of whom are unaccompanied or separated boys -- living in tents in Belgrade, Serbia, the U.N. said.

The U.N. is calling for humanitarian measures to be taken in light of the life-threatening cold facing the refugees and migrants.

Calling the situation in Greece "dire," Sarah Crowe, a spokeswoman for UNICEF, urged government officials in to take the extreme weather into consideration. "It's about saving lives, not about red tape and keeping to bureaucratic arrangements," she said.

Last year, a record of more than 5,000 refugees and migrants died or went missing attempting to reach Europe by the Mediterranean Sea, while some 360,000 arrived on Europe's shores, according to figures from the U.N. Many of the refugees hail from countries like Syria, Iraq and Afghanistan that have been mired in conflict.