Survivors and rescuers are battling the devastation left by sea surges that wiped out entire communities, killing about 23,000 people.
Many do not know if missing relatives are dead or alive
The death toll is still spiralling upwards and mass graves are being dug even as people hunt for the missing.
The extent of the damage is still not known in areas worst hit, including Sri Lanka, Indonesia, India and Thailand.
International aid efforts have begun amid fears that disease could spread through the disaster zone.
Survivors may have little clean water or sanitation as they try to build shelters and bury the dead after Sunday's 9.0 magnitude earthquake sent huge waves from Malaysia to Africa.
"This may be the worst natural disaster in recent history because it is affecting so many heavily populated coastal areas... so many vulnerable communities," UN emergency relief co-ordinator Jan Egeland told CNN.
In northern Indonesia, nearest to the epicentre of the undersea quake, the vice-president said he feared fatalities in the worst-hit province of Aceh could rise as high as 20,000.
Jusuf Kalla appealed for urgent international aid.
"It's very, very bad," he told reporters after a trip to the area. "We need help fast."
The number of dead has also soared well into the thousands in Sri Lanka and India, and thousands more may have been killed on the Andaman and Nicobar islands where reports say entire communities were swept into the sea.
Packed holiday resorts in Thailand were also badly hit, and the waves killed people in Malaysia, the Maldives, Burma and Bangladesh.
Thousands are missing and many more thousands forced from their homes by the worst earthquake in 40 years that generated a wall of water speeding across the oceans.
Hundreds of fishermen are feared drowned off the coast of Somalia, officials said.
Aftershocks have also been detected, sparking warnings from Indian and Sri Lankan weather officials of further, smaller surges, also known as tsunamis.
Sri Lankan rescue workers have been combing the coastline by ship, plane and helicopter, searching for survivors and pulling the dead from the water.
About a million are now homeless.
Click below to see how the disaster unfolded
"The scale of the tragedy is massive and Sri Lanka... has never been hit by tidal waves or earthquakes in its known history," President Chandrika Kumaratunga told the BBC.
Searches have also been continuing off southern India for those swept away from beaches or in fishing boats.
1960 - Chile, 9.5 magnitude
1964 - Alaska, 9.2
1957 - Alaska, 9.1
1952 - Russia, 9.0
2004 - Indonesia, 9.0
In Thailand, bodies were still being taken to makeshift morgues in the resort of Phuket.
Many are said to be clad in swimsuits, with people dragged to their deaths as the tsunami smashed into beaches without warning.
A national disaster has been announced in the low-lying Maldives islands, more than 2,500km (1,500 miles) from the quake's epicentre, after they were hit by severe flooding.
International organisations have already made pledges to help the victims.
- The International Monetary Fund promised "whatever possible assistance"
- The Red Cross launched an appeal for 5m euros (£3.5m; $6.8m)
- The European Union pledged 3m euros (£2.1m; $4.1m)
- Australia promised $10m (£4m; $7.7m) and sent two planes carrying drinking water and purification equipment to Indonesia
- Russia sent 25 tons of humanitarian aid to Sri Lanka
- The UK is sending a plane to Sri Lanka with plastic sheeting and tenting
- France will send humanitarian aid and doctors to Sri Lanka and Thailand.
Sunday's tremor - the fifth strongest since 1900 - had a particularly widespread effect because it seems to have taken place just below the surface of the ocean, analysts say.
Experts say tsunamis generated by earthquakes can travel at up to 500km/h.
IMPACT OF THE EARTHQUAKE