The US plans to increase by 10-fold - to $350m - its contribution to help the survivors of the Indian Ocean tsunami.
Whole communities have been wiped out
The pledge was made just before talks between senior US and UN figures on co-ordinating aid efforts.
The UN says $1.1bn in aid has been pledged so far, for about five million survivors. But relief work appears disorganised, correspondents say.
At least 124,000 people died in the tsunami. The UN says the toll is nearing 150,000 and may never be known.
"The vast majority of those are in Indonesia and Aceh, which is the least assessed area because of logistical constraints, and it may therefore raise further," UN Humanitarian Affairs Co-ordinator Jan Egeland told reporters.
"We will never ever have the absolute definite figure because there are many fishermen and villages which have just gone and we have no chance of finding out how many they were."
Thousands are still missing after a huge undersea earthquake struck off the Indonesian island of Sumatra on Sunday, sending giant waves smashing into coastlines from Malaysia to East Africa.
New Year festivities were cancelled in several affected countries where memorial services were held.
Relief for Aceh
In the Indonesian province of Aceh - which bore the brunt of both the earthquake and the sea surges - the BBC's Rachel Harvey says at last there is some good news.
A US aircraft carrier, the Abraham Lincoln is lowering anchor nearby and 12 Hercules helicopters on board are set to begin flying out aid supplies. The US has also supplied 80 trucks to distribute aid to the area.
Until now there has been little sign of a co-ordinated relief operation for the estimated two million people who have been displaced in Aceh.
The province's ravaged infrastructure has been a major obstacle for aid agencies, says our correspondent - which is why helicopters are so desperately needed.
In Sri Lanka - the second worst-hit country - poor infrastructure also remains a problem for remote mountainous areas.
Some aid is getting through to northern areas held by Tamil Tiger rebels, some of which are co-operating with the government in Colombo.
Other tsunami developments:
- Survivors in the Indian Andaman and Nicobar Islands - where foreign aid organisations have largely been refused access - have complained that authorities have done little to help
- The Indian Express newspaper reports that the Indian air force, based on Nicobar island, alerted mainland authorities to the approaching tsunami an hour before it hit, but that bureaucratic confusion meant nothing was done
- It could be weeks if not months before all those killed in Thailand are identified, rescue teams say
- At least 5,000 Europeans are still missing
- UK individuals' contributions to the aid effort reach £50m ($95m); government raises its pledge from £15m to £50m
US Secretary of State Colin Powell - who is to visit stricken areas on Sunday - said the 10-fold increase in Washington's aid contribution was indicative of the extraordinary need.
Speaking after talks in New York with UN Secretary General Kofi Annan, Mr Powell called on all other countries to make as significant a contribution.
Mr Annan said it was a "a race against time and we are pressing ahead, trying to do it as fast as we can".