BEIRUT — The U.N. children's agency said Friday that 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, said in a statement 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 deaths. 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.

The death of Ali, a severely malnourished 16-year-old boy, as international aid workers were inside Madaya, reinforces the enormous scale of the humanitarian catastrophe in Madaya and other besieged areas.

The U.N. Security Council is scheduled to hold an emergency meeting Friday at the request of Western countries trying to press Syria's warring parties to lift sieges on towns where hundreds of thousands have been cut off from aid and many are starving.

U.N. Secretary-General Ban Ki-moon, who called the deliberate starvation of civilians a "war crime," also stepped up the pressure, calling Thursday for both the Syrian government and rebels to end the sieges before peace talks scheduled for Jan. 25 in Geneva as a confidence-building measure.

Ban said the United Nations and its humanitarian partners are able to deliver food to only 1 percent of the 400,000 people under siege in Syria, down from an already dismal 5 percent just over a year ago.

Juliette Touma, an Amman-based UNICEF representative, said the agency's staff who spent close to seven hours in Madaya Thursday are "terribly shocked."

"In general, they saw pretty horrific scenes of women, children, and elderly, and malnourishment," she told The Associated Press. "It was safe to say the scene was very bleak."

Still, she said there was a sense of relief to finally be able to get into these areas. "It is important right now to maintain this humanitarian access ... There are 14 other Madayas," she said.

Singer, in the statement, said that at the makeshift hospital UNICEF visited in the town, there were only two doctors and two health professionals working under overwhelming conditions.

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Associated Press writer Philip Issa in Beirut contributed to this story.