"In a normal year, that would be a huge statement to the commitment to humanitarian action," Amos told reporters in Geneva of the $5.1 billion raised so far that has gone to people in 24 countries, including Syria, Niger, Sudan and Afghanistan. "But this is an extraordinary year."
The Syrian civil war that's spilling into the region is taking up as much as a third of the needs this year.
Amos, a baroness and British lawmaker who heads the U.N. Office for the Coordination of Humanitarian Affairs, said Syria requires at least $4.4 billion in aid for the 6.8 million people who are suffering inside the war-torn country and the 5.3 million refugees or others who are affected by the conflict in border nations.
She conceded she had "no idea" how to get the rest of the money that is meant to pay for the year's collective humanitarian work of 620 U.N. and affiliated aid agencies.
Adding to the challenge is that global humanitarian aid more broadly fell 8 percent last year to $17.9 billion, down from $19.4 billion in 2011, according to a separate report Wednesday from the British-based anti-poverty organization Development Initiatives. The organization measures global needs, not just those of the U.N. and affiliated aid agencies.
Donors could save money if they responded to crises sooner or spent more on development and other ways of cutting the risk of conflicts, said Judith Randel, the executive director of Development Initiatives.
The biggest previous U.N. request was in 2010 for $11.25 billion which was only 64 percent funded to deal with the Haiti earthquake, Pakistan's floods and other major crises.
But the likelihood is that as much as a third of what the United Nations says is needed this year won't be raised, according to the U.N.'s own figures.
Over the past five years, the United Nations has only been able to scrounge up between 63 percent and 72 percent of what it says is required to feed, house and provide other help to the millions of people trying to survive often desperate conditions, the figures show.
Going back to 2000, the trend looks even bleaker, with the level of U.N. funding requests that have been filled ranging from 55 percent to 72 percent, according to an analysis by Development Initiatives.
"Halfway through the year, we know that 73 million people need humanitarian assistance," Amos said. "The increase is largely due to the exploding crisis in Syria, and the neighboring region, but also because situations have worsened in other countries like the Central African Republic and Mali."