BEIRUT, Lebanon — As families have made the long, frozen, sometimes barefoot trek out of the last shards of Islamic State territory to a refugee camp in northeastern Syria over the past two months, at least 29 children and infants among them have died before reaching help, mainly from hypothermia, the World Health Organization said on Thursday.

At least 18 of them died over the past week, it said, a toll that added to the hundreds of thousands killed over the course of Syria’s civil war, now approaching its eighth year. Millions of people have been displaced, and thousands continue to flee the violence, across distances full of their own dangers.

About 23,000 people have arrived at a refugee camp in the Hasakah region of Syria over the last eight weeks, mostly women and children fleeing the battle to oust the Islamic State from nearby Deir al-Zour province, the W.H.O. said.