A U.N. envoy warned Monday of a "horrifying" humanitarian situation brewing in Syria as non-governmental organizations pledged more than $480 million for refugees on the eve of a major donor conference.

The United Nations aims to raise $8.4 billion at the Syria donor meeting, which starts on Tuesday in Kuwait.

"Failing to meet the required funds risks resulting in a horrifying and dangerous humanitarian catastrophe," Abdullah al-Maatuq, U.N. 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 Organization.

U.N. humanitarian affairs chief Valerie Amos said the donor response at Tuesday's conference "needs to be comprehensive".

She said that 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 U.N. aid agencies have said they remain underfunded and warned they could halt or downsize their operations.

The Third International Humanitarian Pledging Conference to be held on Tuesday will be chaired by U.N. Secretary General Ban Ki-moon and will be opened by Kuwait's emir, Sheikh Sabah al-Ahmad Al-Sabah.

At the first and second conferences, also hosted by Kuwait, pledges of $1.5 billion and $2.4 billion were made, but the United Nations has complained that not all pledges were honored.

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 people have sought refuge in neighboring countries.

"Every day brings more death, displacement and destruction," the U.N. report said.