As the war heads toward a fifth year, some 4.8 million Syrians or around 40 percent of the total 12.2 million people in need of help are difficult to reach, said Assistant Secretary-General for aid Kang Kyung-wha.
Kang told the Security Council that UN agencies were unable in December to deliver food to the Raqa and Deir Ezzor areas controlled by the Islamic State group and to towns besieged by Syrian government troops and opposition forces.
Cross-border deliveries of aid have reached over 700,000 people, but "despite these efforts, needs continue to outpace response," said Kang.
The United Nations is requesting $2.9 billion this year to fund aid operations for Syria, where fighting has led to what Kang described as "one of the worst displacement of people the world has seen in decades."
UN agencies received less than half of the amount requested for last year, leaving "hundreds of thousands" of civilians without aid during the harsh winter months, said Kang.
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"More funding is urgently required," she told the council.
A total of 7.6 million people have been displaced by the fighting inside Syria in addition to 3.8 million refugees who have fled to neighboring countries, mostly Jordan, Lebanon and Turkey.
"This council must find a way to end the conflict in Syria," said Kang, noting that in four years, the number of Syrians in need of assistance had surged from one million to 12 million.
After meeting behind closed doors to discuss the worsening humanitarian crisis, the Security Council called on world governments to support the Syria aid effort.
The 15 members said they were "gravely concerned" by the increasing number of displaced and refugees, and "alarmed" by the impact of winter storms, Chilean Ambassador Cristian Barros Melet told reporters.
He cited an upcoming donors' conference in Dubai as key to securing funding.