The UN weather agency said it forecast below-normal temperatures and heavy snowfall in the next two weeks in the eastern Balkan peninsula, Turkey, the eastern Mediterranean, and Syria, Lebanon, Israel, and Jordan.
“Many children on the move do not have adequate clothing or access to the right nutrition,” said Christophe Boulierac, spokesman for the UN children’s agency Unicef.
Asked if children could freeze to death, he told a news briefing: “The risk is clearly very, very high.”
Children were coming ashore on the Greek island of Lesbos wearing only T-shirts and soaking wet after travelling on unseaworthy rubber dinghies, the charity Save the Children said in a statement.
“Aid workers at the border reception centre in Presevo say there is six inches of snow on the ground and children are arriving with blue lips, distressed and shaking from the cold,” it said.
Below normal temperatures and heavy snowfall equals bad news for refugees: https://t.co/HTd3iIhlxR— Vanessa Parra ✨ (@ParraV) January 19, 2016
It said temperatures were forecast to drop to -20C in Presevo in Serbia and -13C on the Greek border with Macedonia.
Last year, children accounted for a quarter of the 1m migrants and refugees arriving across the Mediterranean in Europe, Boulierac said.
The UN refugee agency UNHCR said a daily average of 1,708 people had arrived in Greece so far in January, just under half the December daily average of 3,508.
Meanwhile, an estimated 3,500 people, mainly women and children, are believed to be held as slaves in Iraq by Islamic State (IS) militants who impose a harsh rule enforced by gruesome public executions, the UN has reported.
The militant group, which also controls large parts of neighbouring Syria, has committed widespread abuses that may “in some instances, amount to war crimes, crimes against humanity, and possibly genocide,” the report said.
The UN Assistance Mission for Iraq and the UN human rights office estimated that 3,500 people were “currently being held in slavery by ISIL”, using another acronym for IS.
“Those being held are predominantly women and children and come primarily from the Yezidi community, but a number are also from other ethnic and religious minority communities,” said the joint report issued in Geneva.
The report detailed executions by shooting, beheading, bulldozing, burning alive, and throwing people off the top of buildings.
It said the UN had information about the murder of child soldiers and had verified reports suggesting that between 800 and 900 children in Mosul had been abducted for military and religious training.
“Even the obscene casualty figures fail to accurately reflect exactly how terribly civilians are suffering in Iraq,” UN human rights chief Zeid Ra’ad Al Hussein said in a statement.
“The figures capture those who were killed or maimed by overt violence, but countless others have died from the lack of access to basic food, water or medical care.”
It said a number of IS child soldiers were killed by the extremists when they tried to flee fighting in the western Anbar province.
Mr Hussein added that the report laid bare the “horror” that Iraqi refugees were attempting to escape when they fled to Europe and other regions.
At least 18,802 civilians were killed and another 36,245 were wounded in Iraq between the start of 2014 and October 31 of last year as Iraqi forces battled IS, the UN report said.