Areas hit by the Asian tsunami could take up to 10 years to recover, the UN secretary general has warned.
Hundreds of thousands of survivors are now in urgent need
Kofi Annan spoke of the "sheer complexity" of the relief effort, which is spread across a dozen nations.
Aid supplies are piling up in regional warehouses, but in some places, heavy rain has provided an extra obstacle to delivering them to outlying areas.
Mr Annan will travel to Indonesia on 6 January to take part in a meeting of world leaders to discuss further aid.
In other developments:
- US Secretary of State Colin Powell and President George W Bush's brother Jeb head for Thailand and Indonesia later on Sunday as part of an American delegation to see the devastation at first-hand
- The United Nations says $2bn has now been raised in aid for the victims, including up to $500m in emergency assistance pledged by Japan
- Aid supplies have been air-dropped onto all of the Andaman and Nicobar islands, according to the military commander in charge of relief operations, after strong criticism from survivors that aid was slow to arrive
- Flash flooding in Sri Lanka is hampering aid distribution and increasing the risk of disease
- US navy helicopters bring relief supplies to Indonesia's Aceh province from an offshore aircraft carrier.
Survivors are still emerging from the ruins left by the huge quake-triggered waves, which struck a week ago.
At least 124,000 people have been confirmed dead around the rim of the Indian Ocean, with Aceh in Indonesia worst-hit.
The UN has warned the final death toll is likely to be more than 150,000 - and may never be known.
Colin Powell will travel to Asia accompanied by Florida Governor Jeb Bush, the president's brother, who has dealt with the aftermath of several powerful hurricanes in his state in recent months.
Clusters of survivors
In Indonesia, aid is finally being distributed, but only in small quantities, says the BBC's Jonathan Head in Aceh.
US military helicopters have visited remote areas to distribute aid.
However, a spokesman for the UN's World Food Programme said that in some coastal villages outside Banda Aceh, helicopter crews had to throw food and other supplies to the ground after desperate mobs prevented them from landing.
The aid operation in Aceh remains without a visible strong leadership.
In this sensitive region - scene of a long-running conflict between Jakarta and separatist rebels - the government is unwilling to allow the US military unfettered access, says our correspondent, and there are few UN personnel on the ground.
But speedy, well-led and co-ordinated action is crucial to save the isolated clusters of survivors spotted on the Sumatran coast and islands, who may have spent a week without food and clean water.
In Sri Lanka, heavy rains have brought flash flooding, cutting off parts of the east and south-east of the island and seriously disrupting aid distribution.
More than 1.7m people will have to be fed for months, says the UN
Fears of an outbreak of waterborne disease among the hundreds of thousands of people thought to be flocking to emergency camps are high.
There are similar fears in Aceh, where heavy rains have also hit.
But there have been amazing stories of survival. The Indonesian Red Cross is reported to have dug a man out from under the ruins of a house in the provincial capital Banda Aceh after hearing his cries for help.
And on India's remote Andaman and Nicobar Islands, a woman who escaped the killer waves gave birth in the forest, AP news agency reported. She named her son Tsunami.
The Indian army says planes have managed to drop supplies of food and water to all the inhabited islands in the remote chain, some of which are only a short distance from the epicentre of last Sunday's earthquake.
The military commander in charge of the relief effort, Gen BS Thakur, said the most remote locations had now been reached.
Six thousand people are officially listed as dead or missing on the islands, while at least 10,000 more are thought to be unaccounted for.
The UN's relief co-ordinator Jan Egeland said the $2bn in aid promised so far, pledged by 40 countries, was more than all the donations made to the UN's other humanitarian appeals in 2004 put together.
"The international compassion has never been like this," Mr Egeland said.
He said the aid effort would focus on feeding one million Indonesians and 700,000 Sri Lankans for months to come.
"The biggest constraints are the logistical bottlenecks," he told reporters at UN headquarters in New York.
"We need to make small, damaged airstrips some of the busiest airports in the world."