"What we see is that the figures may be approaching 150,000 dead. The vast majority of those are in Indonesia and Aceh, which is the least assessed area because of logistical constraints," Egeland said.
"It may therefore raise further.
"We will never ever have the absolute, definite figure because there are many nameless fishermen and villages that have just gone - and we have no chance of finding out how many they were," he said.
Desperate relief workers continued struggling to get food and water to millions of Asia's tsunami victims facing starvation and disease from what the UN has termed an unprecedented global catastrophe.
Official figures for the death toll stood at 125,000 late on Friday.
UN Secretary-General Kofi Annan said half a billion dollars in aid had been promised or delivered for relief operations after deadly seismic sea waves left up to five million people homeless in Asia.
Indonesia, the worst hit among 11 affected nations with almost 80,000 dead, announced that it would host a summit of world leaders on 6 January.
"The meeting will be to discuss the handling of the impact of the earthquake and the tsunami," Foreign Minister Hasan Wirayuda said.
Invitations would be sent to 23 countries, including the 10 member states of the Association of Southeast Asian Nations, and four major international organisations.
Indonesia is among the countries
that may benefit from debt relief
The possibility of debt relief for poor countries devastated by the tsunamis has also become a major topic as world leaders grapple with the enormity of the human and material cost of the disaster.
US Secretary of State Colin Powell has backed the idea of debt relief to stricken countries. French President Jacques Chirac urged the Paris Club group of rich nations to approve a moratorium on debt payments and Canada put in place a unilateral moratorium.
In Indonesia, rescue workers raced to get aid into the devastated province of Aceh amid growing fears tens of thousands more people could quickly die from disease, starvation and injury.
A magnitude 9.0 undersea earthquake off the coast on Sunday sent torrents of water gushing through the exposed province on the northern tip of Sumatra, which accounts for the large majority of the 79,900 people reported dead by authorities in Indonesia.
"The indications are the disaster is going to be a lot worse than we have anticipated already," United Nations Children's Fund (Unicef) communications director, John Budd, speaking from Jakarta said.
"Aceh really is ground zero. There are miles
and miles and miles
Unicef Communications Director
"Aceh really is ground zero. There are miles and miles and miles of nothing."
Budd said there was a desperate shortage of food and fuel across the remote province, which had already suffered from a lack of infrastructure due to a decades-long violent battle between separatist rebels and the government.
"There's no food, there's no fuel, it's a cruel situation. If we get food in, say, rice, there is no pure water or fuel to cook it. We are desperately trying to break this cycle," he said.
The World Health Organisation said that out of an estimated five million people who had been displaced around Asia between one and three million were in Indonesia.
Response to disaster
The death toll from the tsunami
in Sri Lanka may exceed 29,000
On Friday Sri Lanka marked an official day of mourning after cancelling all New Year celebrations with the death toll set to exceed 29,000.
Adding a ghoulish note to the tragedy, local media reported that bodies of tsunami victims in Sri Lanka have been stolen from hospitals and sold to distraught relatives while fingers and ears of corpses have been chopped off to steal jewellery.
Several affected countries, including India where nearly 12,000 people died, have cancelled plans for New Year's Eve celebrations.
The government in Malaysia, where 66 people died, has replaced planned fireworks displays and other celebrations with a call to prayer.