"It must stop," said Yacoub El Hillo, the UN humanitarian coordinator for Syria. "Many more will die if the world does not move faster."
The UN official spoke a day after aid convoys delivered the first supplies in three months to the besieged town of Madaya, where the medical charity MSF says 28 people have died of starvation since December 1.
In Madaya, where government forces have been blocking access for six months, El Hillo said he saw "severely malnourished" residents, especially children who were "extremely thin, skeletal".
El Hillo appealed for UN action to end the sieges, describing the blockades as "the key culprit" for the suffering and a "tactic of war" being used by all sides in the nearly five-year conflict.
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The United Nations says it is struggling to deliver aid to about 4.5 million Syrians who live in hard-to-reach areas, including nearly 400,000 people in 15 besieged areas.
The UN Security Council has adopted resolutions demanding an end to the sieges, but these have been largely ignored.
The patients could be taken to hospitals in Damascus to receive urgent medical aid, he said.
"I have every reason to believe this will happen", he added.
After months of negotiations, the United Nations and its aid partners were able on Monday to send 65 truckloads of food, medical aid, blankets and winter clothing to the trapped residents of Madaya and two other towns.
The deliveries came ahead of a new round of Syrian peace talks planned for January 25 in Geneva.