In one town alone, Bogolay, 10,000 people died when Cyclone Nargis swept across Myanmar last Saturday, flattening 95 per cent of the houses.
Aid agencies say there are serious shortages of power, drinking water and food in the disaster areas, raising fears of disease outbreaks because of poor sanitation and lack of medical supplies.
"Our biggest fear is that the aftermath could be more lethal than the storm itself," said Caryl Stern, US head of Unicef, the UN children's fund.
The densely-populated Irrawaddy delta region took the full force of the storm, and relief agencies say many of the poorest communities there have been virtually wiped out.
Large areas of cropland have also been destroyed in what is the country's main rice-producing region.
Already food prices in Yangon and other parts of Myanmar have soared by up to 100 per cent since the storm hit, adding to the burden of Myanmar's millions of poor.
Paul Risley, spokesman for the World Food Programme in Bangkok, said the UN food agency hoped to step up airborne deliveries within the next 48 hours.
"The challenge will be getting to the affected areas with road blockages everywhere, he said.
As the scale of the crisis becomes clear, Myanmar's military government has made a rare appeal for international assistance.
Nyan Win, Myanmar's foreign minister was quoted on Tuesday by state-run television as saying that at least 10,000 people had died in the delta region, while a smaller number died in and around Yangon, the country's largest city.
|The UN says emergency supplies are urgently |
needed to prevent disease outbreaks [EPA]
"News and data are still being collected, so there may be many more casualties," he said.
Meanwhile, international relief operations are slowly beginning to get access to Myanmar.
The first emergency flight from immediate neighbour Thailand on Tuesday afternoon airlifted in more than $400,000 worth of food and medical supplies.
The European Union has pledged just over $3m in aid, as has the United States, up from an initial emergency contribution of $250,000.
George Bush, the US president, said Washington would make navy warships available for rescue efforts and urged Myanmar to allow international assessment teams into the country.
China and Indonesia have each promised to provide $1m in cash and supplies.
China has long been a staunch diplomatic ally of Myanmar's military government, which is regarded as a pariah in the West because of its human rights record.
Myanmar's military government has previously been suspicious of outside aid offers and have put tough restrictions on the few aid agencies that are allowed to operate in the country.
On Tuesday it announced that a planned national referendum on a new military-backed constitution would be postponed in some of the areas worst-hit by the storm, but would go ahead on schedule in the rest of the country.
The ruling generals have touted the new constitution as a key step on the path to democratic elections in 2010.
But the referendum and the new charter have been dismissed as a sham by pro-democracy groups as well as the US and other Western governments who say they are designed merely to ensure the military's continued grip on power.
The government had been hoping for an easy ride with the referendum, and it remains to be seen what effect the current crisis will have on the vote.
Some opposition groups have suggested the impact of the cyclone could stir an already tense political situation in Myanmar, especially given the government's apparent inability to help its own citizens.