Hundreds of people in the besieged village of Madaya have been told they need to leave or they will starve to death.
The UN humanitarian chief, Stephen O’Brien, said about 400 people in the hospital, near Damascus in Syria, must be evacuated immediately to receive life-saving medical attention “or they are in grave peril of losing their lives”.
After briefing the UN Security Council, he said people in the area needed treatment for medical complications, severe malnourishment and starvation.
Ambulances are expected to get to the village on Tuesday, if a safe passage can be assured.
Nearly 42,000 people in Madaya are at risk from hunger, according to Yacoub El Hillo, the UN's resident and humanitarian co-ordinator in Syria.
The scale of the problem facing the UN and aid workers was illustrated by the comments of Syria's UN Ambassador, Bashar Ja'afari, who has denied anyone is starving.
He blamed Arab television especially “for fabricating these allegations and lies.”
“The Syrian government is not and will not exert any policy of starvation against its own people,” Ja'afari said.
Some Assad supporters also said the photos were faked, and others alleged the rebels were withholding food from residents.
Aid conveys that entered the settlement on Monday confirmed reports that people are starving and have had to eat stray cats and dogs and grass after receiving no aid since October.
The images of people starving and emaciated children have raised global concern after the area has been cut off for months by fighting.
The aid group, Doctors Without Borders have said 23 people have died of starvation at a health care centre it supports in Madaya, since December 1, which included six infants and five people over the age of 60.
Peter Wilson, Britain's deputy UN ambassador, said it was “good news that those convoys are getting through, although it's little and it's late”.
“It's important to remember that Madaya represents only 10 per cent of those who are under siege and 1 per cent of those who need aid in Syria,” he added.
Simultaneously, trucks began entering Foua and Kfarya, which are both under siege by rebel groups hundreds of miles to the north.
It will take several days to distribute the aid in the areas that need it most, and supplies are probably enough to last for a month, aid agencies say.
A group of eight major international aid groups, including Oxfam and Save the Children warned that a one-time delivery would not save starving people.
“Only a complete end to the six-month-old siege and guarantees for sustained aid deliveries alongside humanitarian services will alleviate the crisis in these areas,” a statement by the group said.
The UN says 4.5 million Syrians are living in besieged or hard-to-reach areas and desperately need humanitarian aid, with civilians prevented from leaving and aid workers blocked from bringing in food, medicine, fuel and other essentials.
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