Bodies still littered the streets in north Indonesia two days after the biggest earthquake in 40 years rocked the seabed off the coast, deep below the Indian Ocean, triggering waves up to 10 metres high that battered coasts across the region.
The United Nations said hundreds of relief planes packed with emergency goods would arrive from about two dozen countries within the next 48 hours.
The sheer scale of Sunday's disaster is still unclear amid the chaos with government officials in Indonesia, Sri Lanka and India warning many thousands listing as missing may be dead.
Throughout the region rows of bodies covered in plastic sheets or mats were laid out on the ground.
"I'm tired. I'm looking for my father. Please help me," wailed Maimori, 22, at a market on the outskirts of the Indonesian town of Banda Aceh. Her father was a fishmonger who had not been heard from since going to work early on Sunday.
Tourists were killed on beaches, fishing villages devastated, power and communications cut, and homes destroyed.
Sri Lanka appears to have been the worst hit with about 12,000 dead, with India suffering almost 10,000 deaths.
An Indonesian health official said 5700 had been killed, but Vice-President Jusuf Kalla said on Tuesday he thought the toll could rise to 25,000 and up to 100,000 people mihgt have been injured.
In Thailand, nearly 2000 people - including 700 foreigners - were reportedly dead.
Dozens perished as far away as Somalia, 6000km from the epicentre.
"We cannot fathom the cost of these poor societies and the nameless fishermen and fishing villages ... that have just been wiped out. Hundreds of thousands of livelihoods have gone"
OCHA's Jan Egeland
The finance minister of India said late on Monday that contact had only just been restored with one of the worst-hit islands, Car Nicobar, near the epicentre of the magnitude 9.0 earthquake. The toll there was not known, but would be "very large".
About 45,000 people live on the island, which suffered aftershocks throughout Monday.
The UN said the disaster was unique in encompassing such a large area and so many countries.
Death resonated far beyond even these bounds, with dozens of foreign tourists who had flocked to tropical beaches for the end-of-year holiday season also being killed.
Norwegians, Britons, Italians, Swedes, Americans, Danes, Swiss, Australians, Singaporeans and New Zealanders were among the dead in Thailand, Sri Lanka and the Maldives.
Clean water shortage
"The cost of the devastation will be in the billions of dollars," said Jan Egeland, head of the UN Office for the Coordination of Humanitarian Affairs (OCHA).
Nearly 1000 people have been
killed in Thailand
"However, we cannot fathom the cost of these poor societies and the nameless fishermen and fishing villages ... that have just been wiped out. Hundreds of thousands of livelihoods have gone."
Throughout the region, homeless people fearing another wave sheltered in public buildings, schools and on high ground.
There was a shortage of clean water and provisions.
Weather officials in India warned of more high waves over the next day or two, and urged people to stay away from the shore.
"Like a ripple, the tsunami will only die down gradually and so we expect more waves before they slowly subside," said S Sridharan, of the meteorological department's office in Madras, capital of Tamil Nadu state - one of the worst-hit areas.
Those not searching for survivors hastened to bury the dead, often in mass graves.
In Cuddalore, India, back-hoes were used to dig graves.
Tsunami - Japanese for 'harbour wave' - is usually caused by earthquakes under or near the ocean.
It is not a single wave, but a series of waves that can travel across the ocean at more than 800kph.
As it enters the shallows of coastlines in its path, its velocity slows but its height increases.
A tsunami that is just a few centimetres or metres high from trough to crest can rear up to heights of 50m as it hits the shore, striking with devastating force.
"We must have dug some seven or eight pits and buried 25, 30, 35 bodies in each of them," said grave digger Shekhar.
"We lined up bodies next to each other in two rows and buried them. I've never buried so many in a single day in my life."
Survivors face their greatest danger in coming days.
"The biggest threat ... is from the spread of infection through contamination of drinking water and putrefying bodies left by receding waters," OCHA's Jamie McGoldrick said in Geneva.
The UN's Egeland said there could be epidemics of intestinal and lung infections unless health systems in the stricken countries get the help they need.
"Bigger waves have been recorded, but no wave has affected so many people in this way, because we have had a population explosion since the last tsunami, and some of these areas are among the most populated in the world."