UNITED NATIONS, Feb. 5 (Xinhua) -- UN humanitarians are "gravely concerned" for thousands of people fleeing one of the last Islamic State (IS) strongholds in southeastern Syria, where 35 children have died since last December, Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres, said on Tuesday.

"Our humanitarian colleagues remain gravely concerned for thousands of people fleeing the last (IS)-held areas in Hajin, in Syria's southeastern Deir-ez-Zor governorate, as well as for those civilians who remain in the area," he said. "Those fleeing report many civilian casualties, including women and children, large-scale destruction of civilian infrastructure and shortage of food and medicine."

The spokesman told reporters at a regular briefing that more than 25,000 people have arrived from the Hajin area to the Al Hol camp in Al Hasakeh governorate, effectively tripling the camp's population to some 35,000 people.

"As of yesterday, at least 35 children have reportedly died, either en route or shortly after arriving in the camp since December, mostly from hypothermia," Dujarric said.

Since Jan. 22, around 10,000 people have arrived at the site, he said. The camp's maximum capacity has now been exceeded, he said.

The United Nations and humanitarian partners continue to scale up the response at the camp, he said. Teams have been set up to receive 24 hours a day the newly displaced, quickly identify the most vulnerable cases and provide urgent assistance, including medical and protection support.

"The UN continues to call on all parties to the conflict and those with influence over them to take all measures to ensure the protection of civilians and civilian infrastructure in line with their obligations under international humanitarian law," the spokesman said.