The impact and cost of the Syrian crisis "has exceeded all previous benchmarks," the UN said.
The Syrian appeal — the most money ever requested to help victims of a single conflict — makes up nearly half of the total $12.9 billion the UN is asking for to provide emergency aid to some 52 million people in 17 countries worldwide next year.
"This is the largest amount we have ever had to request at the start of the year," UN humanitarian chief Valerie Amos told reporters in Geneva, adding that amid a range of complex conflicts raging in places like the Central African Republic, Afghanistan and the Democratic Republic of Congo, needs were expected to rise further throughout the year.
Last year, the UN agencies and other humanitarian aid organizations appealed for $8.5 billion for 2013, but upped that figure to $13.4 billion in June.
So far, they have received just $8 billion, Amos said.
Explaining the hike for 2014, Amos pointed to the Syrian civil war, which has raged for 33 months and killed more than 126,000 people, with no end in sight.
That crisis has put an enormous strain on neighbouring countries, where hundreds of thousands of refugees are sheltering in tents as the winter sets in.
Food running out for Syrians
Monday's appeal came as the International Rescue Committee warned in a report that four out of five Syrians are now worried about food running out.
The UN's World Food Programme said almost half of Syria's population of 22 million had unreliable food sources and 6.3 million people needed urgent, life-saving food aid.
The WFP said it aims to help feed more than seven million people inside Syria and in neighbouring countries next year.
"Last year we were talking about one million people (inside Syria) in need," Amos said.
"That figure went up to 6.5 million. It has now gone up to 9.3 million."
There are, meanwhile, 250,000 people in Syria in so-called "siege communities" whom aid workers have not been able to get to in "eight, nine, ten months," and who are lacking food and medicine, Amos told AFP.
In addition, some 2.5 million people live in places difficult to reach on a consistent basis, she said.
Including the current 2.4 million Syrians living as refugees in surrounding countries, more than half of the country's pre-crisis population is in need of aid.
The UN appeal hinted the number of Syrians living as refugees in Lebanon, Jordan, Turkey, Iraq and Egypt could nearly double to 4.1 million by the end of 2014.
In all, 660,000 Syrians will be living in refugee camps by the end of next year, while another 3.44 million will be living in private accommodations, the appeal said.
The countries themselves already put the number at around three million, five times the 588,000 Syrians who were registered as refugees in the region a year ago.
'Most dangerous crisis . . . since WWII'
In addition, around 2.7 million non-Syrians in the host countries are expected to need humanitarian aid due to the impact of the crisis next-door by the end of 2014, up from 1.8 million today.
"This is a tragedy," lamented Antonio Guterres, who heads the UN refugee agency.
He said that the countries in the region need "massive solidarity" from the international community to cope with a conflict that has evolved into "the most dangerous crisis for global peace and security since World War Two."
While hailing the generosity of Syria's neighbours, Guterres acknowledged that he was "disappointed" that European countries were doing so little to help the refugees.
"There is something fundamentally wrong when a Syrian family . . . needs to take a boat with high risk of drowning to get to Europe," he said.
He called for "mechanisms" like visa and resettlement programmes "to allow for people to reach safety without having to put themselves in the hands of smugglers and traffickers, who are some of the worst criminals in today's world."