The number of dead from Sunday's Indian Ocean killer waves is likely to spiral above 100,000, the Red Cross has said.
The true scale of the disaster may never be known
Senior agency official Peter Rees said he thought the toll would rise sharply when victims are counted on India's remote Andaman and Nicobar islands.
About 77,000 people have been confirmed killed in the earthquake and waves.
US President George W Bush pledged to set up an international coalition, with Australia, India and Japan, to co-ordinate the relief effort.
The US earlier said it was more than doubling its pledge of funds to the region to $35m.
The 9.0 magnitude quake happened just off the coast of the Indonesian island of Sumatra early on Sunday, setting off waves that smashed into coastlines as far away as Africa.
Plane loads of supplies have started arriving across the region, as aid agencies strive to bring relief as quickly as possible.
The UN says disease could double the number killed by the waves.
The International Federation of Red Cross and Red Crescent Societies said the death toll from the actual disaster could reach six figures once more remote areas are checked for victims.
More than 500,000 have been reported injured across the region.
"We're facing a disaster of unprecedented proportion in nature," said the federation's Asia-Pacific chief, Simon Missiri.
Many thousands of people are still unaccounted for, notably in the Indian-administered Andaman and Nicobar Islands near the earthquake's epicentre.
Three aftershocks have also hit the islands, which have a total population of 350,000.
Some parts are still said to be cut off from the outside world.
At least 4,000 are known to have died on the islands, but a police chief who has flown over the stricken areas said one in five of the islanders was either dead, missing or injured.
Officials said roads had been washed away and bridges destroyed making it harder for rescuers to move around.
In Tamil Nadu state on the Indian mainland, officials have given up trying to count victims and are concentrating instead on burying the dead and helping survivors.
But there was some good news from Sri Lanka, one of the worst-hit countries, after the government said it had approved a disaster-relief package for hard-hit areas in the north controlled by the Tamil Tiger rebels.
Also, naturalists on the island said large-scale animal deaths appear to have been avoided, with many sensing the approach of the wave and fleeing to high ground.
And there have been some individual happy endings, such as the rescue of a Sri Lankan fisherman spotted at sea by an air force helicopter.
AP news agency quoted Sini Mohammed Sarfudeen as saying he had clung to his capsized ship for three days after the tsunami.
Meanwhile, a Swedish toddler found by a roadside on the Thai resort island of Phuket was re-united with his father at a hospital where both were receiving treatment.
However, thousands of tourists are still unaccounted for on Thailand's Andaman Sea coast.
And the situation remains bleak in the north of the Indonesian island of Sumatra, the nearest landmass to the earthquake's epicentre.
The BBC's Rachel Harvey in Banda Aceh, the capital of the province of Aceh, reports seeing bodies still lying in the rubble in some areas while survivors scavenge nearby for food.
The first ship carrying emergency supplies is reported to have reached the worst-hit Indonesian town of Meulaboh, where 10,000 are feared dead - 10% of the population.
A TV crew in the area said 80% of the west coast town was under water, while nearby Acehjaya is thought to be completely submerged.
Other reports say a 10-15km strip of coastline was flattened.
Relief workers and troops are only now reaching remoter areas of the coast.
And all over the province, there are reports of people hiding in the hills, too scared to come down to the coast.
IMPACT OF THE EARTHQUAKE