More than 200,000 Syrian refugee children are facing a bitter winter of sub-zero temperatures without proper shelter and clothing, Save the Children has warned.
The aid agency is calling for urgent funding to provide refugee families with enough warm clothing to help them survive.
More than 2.5 million people have been displaced within Syria due to the continuing fighting, according to aid agencies.
At least 400,000 registered refugees are now in neighbouring countries, and the UN expects that figure to reach 700,000 by the end of the year.
The true number of refugees is likely to be much higher, as many thousands are unregistered.
Many of the refugees fled their homes during the summer months and now do not have the clothes they need to keep them warm, the charity said.
Mike Penrose, Save the Children's humanitarian director, said: "As winter sets in, families are starting to take increasingly desperate measures to keep warm.
"In the Al Qaem camp in Iraq, children have told us that they haven't washed for more than two weeks because the water is ice cold.
"Unless we can help families get ready for the harsh weather ahead, we could see the weakest and most vulnerable succumb to the cold and associated diseases."
The region's harsh winter will bring torrential rains combined with freezing temperatures. Many refugees are now clustered in camps or living among local communities in desert or mountainous areas, which will be hardest hit by the cold.