The United Nations has launched an appeal to raise $8.4 billion for Syria this year and hopes to receive major pledges at the donor meeting on Tuesday in Kuwait.
"Failing to meet the required funds risks resulting in a horrifying and dangerous humanitarian catastrophe," Abdullah al-Maatuq, UN special envoy for humanitarian affairs, told a meeting of NGOs.
Among the aid groups which gathered on Monday, Turkey's IHH Humanitarian Relief Foundation made the largest pledge of $100 million.
Other donors included Qatar's Red Crescent Society and Kuwait's International Islamic Charity Organisation.
UN humanitarian affairs chief Valerie Amos said the donor response at Tuesday's conference "needs to be comprehensive".
She said the humanitarian situation had deteriorated in Syria with no reduction in violence and children particularly affected.
The aid is urgently required to provide life-saving assistance to half of Syria's population as several UN aid agencies have said they remain underfunded and warned they could halt or downsize their operations.
The Third International Humanitarian Pledging Conference will be chaired by UN Secretary General Ban Ki-moon and opened by Kuwait's emir, Sheikh Sabah al-Ahmad Al-Sabah.
- Not all pledges honoured -
At the first and second conferences, also hosted by Kuwait, pledges of $1.5 billion and $2.4 billion were made. However, the United Nations has complained that not all pledges were honoured.
In a report on the worsening humanitarian crisis in Syria released last week, Ban said devastation from the fighting had left around 7.6 million people internally displaced.
Another 3.9 million have sought refuge in neighbouring countries.
"Every day brings more death, displacement and destruction," the UN report said.
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With the conflict now in its fifth year, almost half of all Syrians have been forced to flee their homes, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
Of the $8.4 billion needed, $5.5 billion is for refugees in neighbouring nations and $2.9 billion for people inside Syria, it said.
"We don't have a specific target (for Tuesday's conference). We hope donors will pledge generously like they have in the previous two conferences," OCHA spokesman Jens Laerke told AFP.
"We aim to raise the $8.4 billion throughout the year and the Kuwait pledging conference will be an important step."
International aid agency Oxfam on Monday criticised the international response to the Syria crisis, saying the funds were woefully inadequate.
- 'Dig deeper' -
"Wealthy countries meeting on Tuesday in Kuwait to pledge aid have a chance to turn this around, but they must dig deeper than last year. Failure to do so will have a devastating effect on millions of civilians in Syria and its neighbouring countries," Oxfam said.
It said less than 10 percent of European nations have pledged their "fair share" so far in 2015 based on the size of their economies, and overall appeals are only 9.8 percent funded.
The UN aid agency for Palestinian refugees, UNRWA, said it wants $415 million to support the 560,000 Palestinian refugees registered in Syria.
"If we do not receive funds immediately at the Kuwait conference, the programme of cash assistance to nearly half a million people will halt in a matter of a few days," UNRWA said in a statement.
Last year was the deadliest yet in the conflict, with at least 76,000 people killed out of a total of more than 215,000 since it began in March 2011 with peaceful pro-democracy demonstrations.
Since then, 11.4 million Syrians have fled their homes and nearly four million have left the country in what the UN has called the worst refugee crisis in 20 years.
Nearly 10 million people inside Syria do not have enough to eat, and more than 11 million urgently need clean water, UN reports say.
The UN children's fund (UNICEF) says up to two million children are living in areas of Syria largely cut off from humanitarian aid and about 2.6 million are out of school.