The UN estimates that about 9.3 million people in Syria, or about 40 percent of the population, need humanitarian assistance owing to the country's ongoing civil conflict.
Valerie Amos, the UN humanitarian chief, told the 15-member Security Council on Monday that 9.3 million people now need outside help to survive, up from 6.8 million in September, and 6.5 million are now homeless inside the country, up from 4.25 million.
The population of Syria is about 23 million.
"The humanitarian situation inside Syria continues to deteriorate rapidly and inexorably," she said during a closed-doors meeting, according to her spokeswoman Amanda Pitt.
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She said: "Amos continues to press the council for their help and influence over those parties who can ensure the protection of civilians and civilian facilities; the safe passage of medical personnel and supplies; the safe and unhindered delivery of humanitarian assistance; and can facilitate progress in expanding critical, life-saving relief operations."
Amos' plea to the Security Council follows the Syrian government's promise on Monday to ensure delivery of vaccinations and humanitarian aid across the country, after an outbreak of polio in the northeast and warnings of malnutrition in areas under military siege.
Twenty-two children in Deir Ez-zor province bordering Iraq were left paralysed last month.
The polio virus has been confirmed so far in 10 of them, and experts say it could spread quickly across the region.
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Last month, Amos demanded stronger action by the Security Council to get desperately needed aid into Syria, where millions of people in need have not received any help for almost a year.
Violence and excessive red tape have slowed aid delivery to a trickle in Syria. More than 100,000 people have been killed in the civil war and millions have fled the country.
After months of talks, the Security Council approved a non-binding statement on October 2 urging increased humanitarian access.
Amos has complained that that statement has had little impact on the ground.
Senior UN diplomats say that Sergey Lavrov, Russian foreign minister, previously dismissed the possibility of a legally binding resolution on aid access in Syria.