Burmese cyclone survivors face a massive crisis unless they are urgently delivered aid, leading aid agencies have warned.
A likely death toll of 100,000 could rise to 1.5m without provision of clean water and sanitation, Oxfam said.
The International Rescue Committee said that without a massive delivery of aid, Burma faced "unimaginable tragedy".
Eight days after Cyclone Nargis struck, the UN estimates only a quarter of survivors have received any aid so far.
The military government is still refusing to allow many foreign nationals into Burma to distribute relief.
'Perfect storm'
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She added that the disaster was a "perfect storm" which bore "all the factors" for a "public health catastrophe".
The UN, which has launched a $187m (Ј96m) appeal for aid, says those in the worst-affected areas urgently need food, shelter and medical aid.
Deliveries of aid to the country have been arriving sporadically, with correspondents saying some aid is reaching survivors, but not nearly enough.
"Unless there is a massive and fast infusion of aid, experts and supplies into the hardest-hit areas, there's going to be a tragedy on an unimaginable scale," said the US-based IRC's Greg Beck.
But Andrew Kirkwood, the head of the UK aid organisation Save the Children in Burma, told the BBC aid has started to reach an increasing number of people over the last two days.
Burma's state media says 23,335 died, but the UN says the toll could be about 100,000.
Government control
On Saturday, a convoy from the UN refugee agency, UNHCR, crossed into Burma from Thailand with 22 tonnes of tents and other humanitarian supplies.
Joe Lowry, of the International Federation of Red Cross and Red Crescent Societies, said he hoped another seven flights would reach Burma before Monday.
Since the cyclone struck on 3 May, aid agencies already in the country have started relief efforts with supplies they had available and by buying from local sources.
But they warn that supplies will run out unless more aid is allowed into the country.
Christian Aid's Burma expert Ray Hasan said: "Partners are telling us that there are outbreaks of disease already. There is no time to lose."
Aid has been flown in from Burma's neighbours, such as China and Thailand, and the first US relief flight is expected to arrive on Monday.
But aid agencies say the Burmese government does not have the capacity to handle the scale of the relief efforts needed and must allow more foreign aid and disaster experts into the country.
Richard Horsey, a spokesman for UN humanitarian operations said an international presence was needed in Burma to look at the logistics of getting boats, helicopters and trucks into the worst-affected delta area.
"That's a critical bottleneck that must be overcome at this point," he told the Associated Press, speaking from Bangkok.
Disease
The International Federation of Red Cross and Red Crescent Societies said thousands of people were homeless and are living in pitiable conditions.
Hospitals, schools and other large buildings are crammed with the displaced.
Federation workers said they had seen evidence that some aid, such as newly-installed or repaired pumps, were working, but the water around and near these buildings was black-brown and foul-smelling.
Christian Aid says people are asking for rice seeds, as their supplies have been damaged. Unless people plant the seeds in the next month, they will have no supplies of rice until May 2009.
Poll doubts
Meanwhile, Burma's government said there was a massive turnout in Saturday's constitutional referendum.
The UN said it should not have been held at a time of such devastation.
Voting took place across two-thirds of the country, but was postponed for two weeks in the worst-hit areas - including the Irrawaddy delta and Rangoon, the main city.
The country's ruling generals say the vote will pave the way for democratic elections in 2010, while the opposition says it is intended to entrench military rule.
Correspondents say many people in Burma are cynical about the vote and had felt compelled to vote "yes" because of the presence of soldiers around polling stations.
Groups involved in last year's pro-democracy protests accused the junta of concentrating on a "sham constitutional referendum" instead of "putting all resources toward saving the lives" of cyclone victims.
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(BBC)
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