The U.N. children's agency says it has witnessed cases of severe malnutrition among children in a besieged Syrian town and the death of a teenager "in front of our eyes."

Hanaa Singer, UNICEF representative in Syria, says in a statement Friday that the 16-year-old identified as Ali passed away Thursday in the town's clinic of severe malnutrition.

Trucks from the U.N. and other humanitarian organizations entered Madaya Thursday for the second time in a week after reports of starvation. The town has been under siege for months by government forces.

Two other communities, Foua and Kfarya, in northern Syria besieged by Syrian rebels were also included in the aid operation Thursday.

Singer said the people the agency met in Madaya were "exhausted and extremely frail."