Syria conflict: Vital aid 'not reaching most Syrians' - UN
The UN says it has been able to deliver aid to only a fraction of the one million Syrians it planned to reach this month, calling the situation "horrendously critical" in some areas.
UN humanitarian coordinator Jan Egeland said officials had so far only managed to get aid to 160,000 people.
Children in at least three areas were so malnourished they would die if they do not get aid, he said.
Mr Egeland called on all sides to allow humanitarian access.
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There had been hopes of an increase in aid deliveries to besieged areas when the Syrian government and opposition groups agreed a partial ceasefire in February.
But the cessation of hostilities proved fragile and there has since been a surge of violence in the country.
Mr Egeland said delivering aid had been far harder than anticipated.
"Even in areas where we had full approval from the government, there have been infinite problems in actually reaching the places, and in others where we had conditional approvals, like Darayya, we haven't been able to reach the people at all," he said.
He warned that children were close to dying in Darayya and Muadhamiya - rebel-held suburbs of the capital, Damascus - and al-Wair near Homs.
The UN believes that of the 4.5 million people living in what it terms "hard-to-reach" areas of Syria, nearly 400,000 are besieged.
It has already resorted to air drops of food to reach 110,000 people in towns held by so-called Islamic State militants, and says it is considering doing the same for all areas in need if ground access to besieged areas continued to be denied.