The plight of Madaya and other besieged areas has prompted the UN Security Council to call an emergency meeting for Friday, amid warnings that the use of starvation as a weapon constitutes a war crime.
A mobile clinic with medics on board was dispatched to Madaya on Friday to treat people suffering from malnutrition, the World Health Organisation said, a day after a second aid convoy reached the town.
Madaya's 40,000 inhabitants have been living under a crippling siege by pro-government forces that has made even bread and water scarce for months.
Hundreds need urgent evacuation from besieged Syria town
More than two dozen people have reportedly died of starvation since early December.
A teenage boy became the latest victim of hunger, the UN's child agency said.
"The UNICEF team, which included a doctor, witnessed on Thursday evening in a makeshift clinic the death of Ali, a 16 year old, who was suffering from severe malnutrition," said UNICEF spokesperson Juliette Touma.
"It was sad and shocking," she told AFP.
Another 17-year-old boy in a "life-threatening condition," and a pregnant woman who will give birth soon, are both "in urgent need of evacuation," UNICEF said.
There are an estimated 20,000 children living in Madaya, according to UNICEF.
At least 22 children under five showed signs of moderate to severe malnutrition, it said.
The UN agency said Madaya's doctors were "emotionally distressed and mentally drained, working round the clock with very limited resources".
"It is simply unacceptable that this is happening in the 21st century," it said, adding that 14 other besieged and starvation-hit areas existed across Syria.
A convoy of 44 aid trucks loaded with food and medicine on Thursday entered Madaya, where the UN says hardships are the worst seen in Syria's nearly five-year war.
UN Secretary General Ban Ki-moon warned that any forces using starvation as a tactic of war in Syria would be guilty of war crimes.
"Let me be clear: the use of starvation as a weapon of war is a war crime," Ban told reporters.
"All sides -- including the Syrian government which has the primary responsibility to protect Syrians -- are committing this and other atrocious acts prohibited under international humanitarian law," he said.
Syrian authorities have repeatedly denied that starvation is taking place in Madaya.
With mounting international pressure, France, Britain and the United States called for an emergency meeting of the Security Council to push demands for an end to sieges.
French Ambassador Francois Delattre told AFP the meeting, to be held Friday from 2000 GMT, "will draw the world's attention to the humanitarian tragedy that is unfolding in Madaya and in other towns in Syria."
Fuaa and Kafraya, two government-held villages in northwest Syria, have been under siege by rebel groups for months.
On Thursday, about 17 trucks delivered aid to Fuaa and Kafraya's residents, including 6,000 children.
Those two villages, along with Madaya and the nearby rebel-controlled town Zabadani, are part of a six-month truce deal that foresaw an end to hostilities in exchange for aid deliveries.
The UN said the next delivery would take place on Sunday.
More than 260,000 people have died in Syria's conflict, which began in March 2011 with anti-government protests but has evolved into a multi-sided civil war.
Humanitarian aid access is seen as a key confidence-building measure ahead of a new round of Syrian peace talks due later this month.
Russia, which is carrying out a bombing campaign against rebels to support its ally President Bashar al-Assad, said it had launched "humanitarian operations" in Syria, claiming that inhabitants were returning to a "peaceful life" there.
23 die of starvation in beseiged Syrian town: MSF
"In this context, the implementation of humanitarian operations will be a new line of work for the Russian armed forces in Syria," said senior military official General Sergei Rudskoi.
Moscow also revealed that it had signed an agreement with Syria in August giving it the right to retain an open-ended military presence there.
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