* Donors promise large pledges at Wednesday meeting
* U.N. denies opposition charges that Syria controlling aid
* "Unrelenting flow" of refugees across borders (Adds U.S. statement on aid)
By Dominic Evans
BEIRUT, Jan 29 (Reuters) - The United Nations has received promises of major donations at this week's $1.5 billion aid conference for millions of Syrians affected by nearly two years of conflict, a senior U.N. official said on Tuesday.
Wednesday's pledging conference in 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.
"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," said Robert Watkins, U.N. humanitarian coordinator in Lebanon, which has seen the biggest influx of refugees from the Syrian bloodshed.
"We have received preliminary pledges from a number of important donor countries that they will be making announcements of large donations."
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 the Gulf state of Kuwait could encourage other wealthy Gulf Arab states, who have led regional opposition to President Bashar al-Assad, to support the international aid effort.
Many Gulf states have sent assistance, but aid workers in the region say their efforts have been haphazard and rarely coordinated with other aid agencies, hampering their ability to plan a sustained relief programme.
"It's important that we widen the range of donors that provide assistance," Watkins said.
Syria's main opposition coalition has criticised the U.N. appeal and its arrangements for distributing aid inside Syria, saying the organisation has effectively ceded control to the Syrian government and failed to deliver all but a bare minimum of aid to areas controlled by Assad's opponents.
But Watkins said the Syrian government did not influence the aid distribution, although there were some areas of the country which could not be reached because of the violence.
"While we are not able to reach all of the people in need in Syria - because there are 4 million people who need assistance inside the country - we have been able to feed up to 1.5 million people," Watkins said.
"Of those 1.5 million people, 49 percent are in areas which are either under the control of the opposition or in contested areas," he said, adding that the main U.N. aid distribution partner in Syria - the Syrian Arab Red Crescent (SARC) - was "neutral and impartial".
The opposition says the top members of the SARC are close to Assad's government, and the organisation cannot operate even-handedly. But activists say its workers on the ground have come under fire from both sides in the conflict.
Syria's uprising began in March 2011 as a peaceful protest movement, but rebels took up arms after the government cracked down on the demonstrations.
The rebellion has since become a full-scale civil war. Insurgents have taken swathes of rural territory from government forces but have failed to capture major cities and towns. More than 60,000 people have died, the United Nations says.
The number of refugees fleeing to neighbouring countries has jumped sharply in the last two months, passing the 500,000 mark on Dec. 11 and growing by more than 200,000 since then.
On Monday, 3,000 Syrians fled into Jordan.
"We have seen an unrelenting flow of refugees across all borders. We are running double shifts to register people," Sybella Wilkes, spokeswoman for the U.N. High Commissioner for Refugees (UNHCR), told Reuters in Geneva.
Jordan has 171,033 registered Syrian refugees, as well as 51,729 who await processing, many of who fled fighting around the southern Syrian town of Deraa this month. Lebanon has 158,973 Syrian refugees, and 69,963 awaiting processing.
"We are trying to clear a backlog of people because the numbers have gone up so dramatically (in Jordan and Lebanon)," Wilkes said.
Turkey has 163,161 Syrian refugees in its 15 camps while Iraq hosts 77,415, the UNHCR said. There are 14,375 in Egypt and 5,417 registered across the rest of North Africa. (Additional reporting by Stephanie Nebehay in Geneva)