"This is the largest ever appeal for a single crisis," Amos said.
In Syria, there are 2.5 million people in "hard to reach communities" because of fighting and security concerns, she said.
The nearly 3-year-old conflict has claimed more than 100,000 lives and displaced millions, and even if it ends tomorrow the humanitarian aid would still need to continue, Amos said.
"We're facing a terrifying situation here where, by the end of 2014, substantially more of the population of Syria could be displaced or in need of humanitarian help than not," said the U.N.'s High Commissioner for Refugees António Guterres.
"This goes beyond anything we have seen in many, many years, and makes the need for a political solution all the much greater," he said.
A year ago, the U.N.'s humanitarian request looking ahead to 2013 was for $8.5 billion, but Syria's civil war forced the world body to revise that assessment upward to $13.6 billion. U.N. and other aid officials said Monday that their 2013 request will be only 60 percent funded.
Such funding gaps will leave many people hungry, lacking shelter and unprotected from violence, they said.
"When looking ahead to the 2014 plans for humanitarian response and the funds that are required, the NGO community is very concerned that this year's appeals are still vastly underfunded and that leaves gaps in meeting immediate humanitarian needs and also slows down the recovery process for millions of civilians, leaving an unbearable scar for years to come," said Jasmine Whitbread, CEO of Save the Children International.
She said the 2013 requests for Haiti, Somalia, Djibouti, Central African Republic, and the Philippines "are all less than 50 percent funded."
Speaking from the Lebanese border town of Arsal, U.N. refugee agency spokesman Peter Kessler told The Associated Press that the focus in 2014 will shift to helping host countries develop and upgrade their infrastructure to accommodate the large number of refugees.
Unlike in Turkey, Jordan and Iraq, the Lebanese government is not providing facilities or land to temporarily accommodate refugees, despite the continuing influx into the country of 4.5 million. The refugees are scattered all over the country, and mostly live in informal tented settlements in the north and in the Bekaa Valley.
More than 20 percent of Lebanon's inhabitants are now Syrians, who have fled the fighting, Kessler said. He estimated that at least 120,000 Syrians seek shelter in neighboring countries every month.
"That means that thousands of people need blankets, clothes, health care, education and a decent shelter to live in," Kessler said. "The needs are enormous and the host countries cannot meet them on their own. They need help," Kassler said.
The U.N. acknowledges that its request for 2014 is formidable, "but attainable."