KUWAIT: The United Nations is receiving only limited support for its aid to millions of Syrians, Secretary General Ban Ki-moon said in published remarks, adding its humanitarian work needed "generous" help from a donor pledging conference on Wednesday.

The gathering in the Gulf Arab state of Kuwait will seek $1 billion of aid for Syria's neighbours sheltering 700,000 registered refugees, and another $500 million to bankroll humanitarian work for 4 million Syrians inside their country.

So far, the United Nations has received pledges covering just 18 percent of the target, unveiled last month as the scale of Syria's humanitarian crisis escalated sharply, and which aims to fund operations for the first half of this year.

Ban was quoted by the official Kuwait News Agency (KUNA) as saying what while the need for humanitarian aid was rising "the level of response the U.N. is receiving is very much limited."

"That is why I am appealing to the whole international community to render their generous support."

Some 4 million Syrians inside the country need food, shelter and other aid and more than 700,000 more have escaped to neighbouring countries since the 22-month-old conflict began, according to the U.N.

KUNA reported Ban as saying that on a visit to refugee camps in Jordan and Turkey six weeks ago he heard stories of refugees who had fled Syria "and particularly stories from children, who were very much concerned about their own future."

"That really saddened and humbled me."

Robert Watkins, U.N. humanitarian coordinator in Lebanon, which has seen the biggest influx of refugees from the Syrian bloodshed, told Reuters that the United Nations had received promises of major donations at the Kuwait conference.

"We have every reason to be optimistic that there will a very good presence and new pledges that will be coming up at this conference," he said.

U.S. President Barack Obama announced an additional $155 million, bringing the total U.S. humanitarian aid to the Syrian crisis to some $365 million, the State Department said.

Watkins said the fact that the conference was being held in Kuwait could encourage other wealthy Gulf Arab states, who have led regional opposition to President Bashar al-Assad, to support the international aid effort.

In New York, U.N.-Arab League mediator Lakhdar Brahimi warned the U.N. Security Council on Tuesday that Syrian President Bashar al-Assad may be able to cling to power for now but the country is "breaking up before everyone's eyes," diplomats told Reuters.

Brahimi suggested that attempts to end the 22-month-old conflict, which has claimed more than 60,000 lives according to U.N. figures, had not progressed in the last two months. He said it was up to the Security Council to end its impasse.