Condemning Syria's "barbaric" sieges, the United Nations demanded immediate access to besieged towns to deliver food, medicine and other life-saving aid to civilians facing starvation.
"There can be no reason or rational, no explanation or excuse, for preventing aid from reaching people," UN aid official Kyung-Wha Kang told an emergency Security Council meeting on ending the blockades.
France and Britain requested the urgent talks after reports emerged of dozens of people who have died from starvation in the town of Madaya, where aid deliveries finally arrived this week.
A total of 35 people have died there since early December, according to medical charity Medecins Sans Frontieres (MSF), which warned a dozen more patients "could die very soon if they are not evacuated."
Madaya's 40,000 residents have been living under siege by pro-government forces for months.
"The barbarity of this tactic cannot be overstated," Kang told the council.
"You cannot let more people die under your watch."
A mobile clinic with medics was dispatched to Madaya to treat people suffering from malnutrition, the World Health Organization said, a day after a second aid convoy reached the town.
A teenage boy, 16-year-old Ali, became the latest victim of hunger.
Ali's death late Thursday was witnessed by representatives of the UN children's agency, UNICEF, as they assessed the health situation of residents of the famine-stricken town.
"They took us down to the makeshift hospital and we went to the basement" where two young men shared a bed, UNICEF's top Syria representative, Hanaa Singer, told AFP.
Singer said the two boys' bodies "were skeleton-like."
A UNICEF doctor approached one of the teenagers who looked particularly weak and noticed his pulse had stopped.
"She checked him out, there was no pulse, so she started resuscitating. One, two, three times, then she looked at me and said, 'He's gone.' And she closed his eyes," Singer said by phone from Syria.
So far, nine people have been allowed to leave Madaya to receive treatment and 19 others are in need of urgent evacuation, said Kang.
- Starvation 'war crime' -
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 were guilty of a "war crime."
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"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 told reporters.
Syrian authorities have repeatedly denied that starvation is taking place in Madaya.
On Friday, the mobile clinic provided preliminary medical services to Madaya residents and returned to Damascus in the afternoon, according to the Syrian Arab Red Crescent's head Tamam Mehrez.
The United Nations is struggling to deliver aid to about 4.5 million Syrians who live in hard-to-reach areas, including nearly 400,000 people in besieged areas.
- Unbearable suffering -
In his address to the council, French Ambassador Francois Delattre recalled that aid workers who reached Madaya had provided accounts of the "unbearable" suffering in the town, and that now, "no one can say we didn't know."
Of the 91 current requests from the United Nations for aid deliveries, only 13 have been granted, he said.
"Madaya is just the tip of the iceberg," said British Deputy Ambassador Peter Wilson, warning that the images of emaciated children could be repeated "many, many times over."
"We cannot allow this to happen," he said.
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 planned for January 25 in Geneva.
Syrian ally Russia complained that much attention was focused on Madaya, with Deputy Ambassador Vladimir Safronkov saying "not a word" was said about the other towns under rebel siege.
On Thursday, about 17 trucks delivered aid to residents of Fuaa and Kafraya, two government-held villages in northwest Syria under siege by rebel groups for months.
The UN said the next aid delivery would take place on Sunday.
Russia, which is carrying out a bombing campaign against rebels to support President Bashar al-Assad, said it had launched "humanitarian operations" in Syria.
In the United States, President Barack Obama met with his National Security Council to discuss "intensification" of Washington's campaign against the so-called Islamic State group in Syria and Iraq.
Obama directed his team to fight IS harder by "working with our partners to increase our military cooperation, disrupting foreign fighter networks, halting ISIL expansion outside of Syria and Iraq, countering ISIL financing, disrupting any ISIL external plotting efforts and countering ISIL's propaganda and messaging," the White House said, using an alternate acronym for the IS group.
"The United States is strongly committed to continuing to lead the shared efforts of the Global Coalition to Counter ISIL."