Andrej Mahecic of UNHCR said in Geneva that malnourishment and hypothermia have been the principal causes of the 29 children's deaths. They are among some 10,000 people who have fled the area near the Iraqi border and reached the al-Hol tent settlement in Hassakeh, raising its population to more than 23,000, Mahecic said.

The evacuations from eastern Syria come during frigid winter weather in the desert region, with mostly women and children fleeing amid the fighting there.

The U.S.-backed Kurdish-led Syrian Democratic Forces have captured wide areas from IS in recent months in the eastern province of Deir el-Zour. The extremists are now besieged in a small pocket near Iraq's border. The fighting that began with an SDF offensive on Sept. 10 has left hundreds dead on both sides. The extremists now control only two villages.

As IS loses ground, thousands of people, many of them women and children, are fleeing and most of them are being taken to a tent settlement in the northeastern province of Hassakeh.

Syria's conflict, which began in March 2011, has impacted children heavily, many of whom have been killed or wounded. The Britain-based Syrian Observatory for Human Rights, a war monitor, released a death toll for the conflict in December saying that among the half a million people killed over the past seven years, 20,819 were children or teenagers.

Mahecic said the children who have died since early December — including newborns — have died both during their journey and shortly after their arrival. Medical facilities in Hassakeh, where the most critical cases are referred to from the camp, are overstretched caring for acutely malnourished children.

He said UNHCR and partners have set up 24-hour response teams to receive the newly displaced people, quickly identify the most vulnerable cases and provide urgent assistance, especially to unaccompanied or separated children and those who require immediate medical assistance..

Mahecic said UNHCR and humanitarian partners are racing to meet the urgent needs of vulnerable civilians at al-Hol. He said families fleeing the IS-held enclave and surrounding areas have also told UNHCR "of a harrowing journey to safety."

"They travel at night with barely any belongings, often having to wade through mine fields and open fighting," he said.

Mahecic added that on reaching SDF positions they describe "being herded into open trucks and having to endure another arduous journey in winter weather northwards to al-Hol."

"Little or no assistance is provided en route to the hungry and cold people, the vast majority of whom are women and children," he said.

The Britain-based Syrian Observatory for Human Rights, a war monitor, said many of those in al-Hol are Iraqi citizens. It added that the dead children include eight Iraqis killed by fire caused by "primitive" methods used for heating.