The UN has called for greater humanitarian access in Syria amid fears that many civilians risk starvation in besieged areas of the war-torn country.
Speaking in Geneva on Thursday, Staffan de Mistura, the UN special envoy, said there "are plenty of civilians at the moment in danger of starvation" in Syria, where more than 400,000 are trapped in areas besieged by the government or armed groups across the country.
The UN also estimates that upwards of four million people are trapped in "hard-to-reach" areas.
Jan Egeland, secretary-general of the Norwegian Refugee Council and special adviser to the UN envoy, said plans to reach a million Syrians with humanitarian assistance have fallen short.
"Of the one million people that we have planned and have tried to reach by land in May, we've only so far reached 160,000," he said.
"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 Daraya, we haven't been able to reach the people at all."
Daraya and Moadamiyah near Damascus and al-Waer in Homs were the three places where the situation was "horrendously critical", he said.
"Children are so malnourished in these places that they will be dying if we're not able to reach them."
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The UN has resorted to airdrops of food to reach 110,000 people besieged by the Islamic State of Iraq and the Levant (ISIL, or ISIS) group in the town of Deir Az Zor, and is considering airdrops to places besieged by government forces if it does not get permission to go in by land.
De Mistura said those airdrops would still need government approval, but if that was denied, he expected the United States and Russia to find a way to ensure everyone could be reached.
The five-year Syrian conflict has killed more than 270,000 people in the past five years, according to the UK-based Syrian Observatory for Human Rights.
De Mistura said that he has no plans to convene a new round of talks in the next two or three weeks as fighting flares on the ground.
He told a closed session of the Security Council that more progress was needed to strengthen a ceasefire and deliver humanitarian aid before talks can resume.
The envoy "briefed on his intention to start the next round of talks as soon as feasible but certainly not within the next two/three weeks," said a statement from his office.
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Two weeks of UN-brokered talks between the Syrian government and opposition groups in Geneva ended on April 27 with no breakthrough.
A new round had been expected for the end of May, but no new date has been announced.
Time is running out before an August deadline for the peace talks, and some diplomats had expected the timetable would be even tighter because talks might not be scheduled during the Muslim holy month of Ramadan, which starts on June 6.
But de Mistura said earlier on Thursday that Ramadan would "not be a factor", saying that if people were able to keep fighting during Ramadan, they could be expected to conduct peace talks.