Some 9.3 million people in Syria – or about 40% of the population – now need outside assistance, UN humanitarian chief Valerie Amos has said.
This figure has risen by 2.5m from the 6.8m total the UN gave in September.
The Syria crisis “continues to deteriorate rapidly and inexorably”, Ms Amos told the UN Security Council.
Meanwhile, UN-Arab League envoy Lakhdar Brahimi is due to hold talks with US and Russian diplomats aimed at paving the way for a Syria peace conference.
In Geneva, Mr Brahimi will also meet representatives from the rest of the UN Security Council and Syria’s neighbours ahead of the conference planned for later this month.
The Syrian government and opposition groups disagree over how the formal negotiations should be structured: the opposition demands that President Bashar al-Assad should resign, while Damascus says there should be no pre-conditions.
In a separate development, Damascus announced a nationwide vaccination campaign to to immunise every Syrian child against polio, measles, mumps and rubella, even in rebel-held territories.
This comes weeks after cases of polio were confirmed in the country for the first time in 14 years.
Baroness Amos said the UN Security Council “should put its full political weight with both the regime and opposition parties” to ensure access for humanitarian workers.
The baroness “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,” her spokeswoman Amanda Pitt said.
More than half of those in need are people living in Syria displaced by conflict, a total of 6.5m, up from 4.25m internally displaced people in June.
Last month, the UN Security Council expressed alarm at the “rapid deterioration” of the humanitarian situation in Syria and demanded immediate access for aid.
It followed a resolution on eliminating Syria’s chemical weapons.
The UN says the number of those needing help has risen by more than 30% from 6.8 million in September. Syria has a population of 23 million.
There are more than 2.5 million people in isolated or besieged areas of Syria, many of whom are living without adequate food or electricity or access to medical supplies.
Aid agencies complain that the Syrian government has hindered access to visas and tried to limit the number of foreign groups operating in the country.
Neighbouring countries such as Jordan are warning that they cannot cope with the influx of refugees crossing from Syria.
The UN estimates that more than 2m people have fled Syria since the unrest began in March 2011 resulting in a humanitarian crisis.
Most have sought refuge in Lebanon, Jordan, Turkey, Iraq and Egypt.
More than 100,000 people are estimated to have been killed since the conflict began.