Five-year-old Intissar and her younger sister Lin were sheltering from northern Syria's bitter winter cold when fuel from a heater ignited their tent, killing them and seriously injuring their mother.
The young family and other displaced Syrians were living near the Turkish border in a camp of more than 400 tents, which offer little protection from snow storms and plunging temperatures which struck in recent days.
The cold snap has brought chaos to traffic and flights in neighboring countries but its effects are most severe in northwest Syria, where 3 million people have been left homeless in a long-running humanitarian crisis.
Many have been displaced several times by the 11-year war.
"People in the camp are suffering. The tents don't protect from the cold," said Nouredin al-Abdullah, whose cousin Ahmed is the father of the girls who died. "If you think about heating, God forbid, you and your children may go (the same way)".
He said the latest snowfall was the heaviest he had seen. The weight of the snow has collapsed many tents, while water seeped underneath them.
Across the region, food supplies and health services have been disrupted and relief workers are struggling to reach some of the 300 worst affected sites, said Mark Cutts, UN deputy regional humanitarian coordinator for Syria.
"The numbers are just staggering, and it is very difficult to provide people with all the support they need," he said. Desperate to stay warm, people were burning cardboard and plastic bottles, and then inhaling toxic fumes.
"Even more suffering is caused because of the lack of fuel for heating," he told Reuters, adding that at least one child had died from the freezing cold.
"There are more than 1 million people still living in tents or substandard accommodation," Cutts said. "It's becoming increasingly urgent that we get people out of these tents."