BANGKOK, THAILAND - An estimated 1.5 million Myanmarese are on the brink of a "massive public health catastrophe," the British charity Oxfam warned Sunday, as desperate survivors of Cyclone Nargis poured out of the devastated Irrawaddy Delta into regional towns in search of water, food and other help.

Myanmar faces conditions that could lead to an outbreak of waterborne disease, said Sarah Ireland, Oxfam's regional director.

"The ponds are full of dead bodies, the wells have saline water, and even things like a bucket are in scarce supply," Ireland said. She appealed for Myanmar authorities, who have restricted access to the country, to allow humanitarian agencies to send in technical and health experts to help prevent disease outbreaks.

The struggling relief efforts suffered another setback when a boat ferrying rice, drinking water, clothing and other aid supplies sank in the delta early Sunday, apparently after hitting a submerged tree, the International Federation of Red Cross and Red Crescent Societies said.

Residents were able to salvage some of the supplies, meant for more than 1,000 survivors, but river water probably contaminated the food, the organization said. All of those aboard made it safely to land. The boat was carrying one of the first international aid shipments.

"This is a great loss," said Aung Kyaw Htut, who is supervising the distribution effort. "This would have been our very first river shipment, and it will delay aid for a further day."

In what was seen as a huge concession by the junta, the U.S. finally got the go-ahead to send a C-130 cargo plane packed with supplies to Yangon today, with two more air shipments scheduled to land Tuesday, The Associated Press reported.

The cyclone and powerful tidal sea surge ripped across the low-lying delta a week ago. The country's ruling junta on Sunday raised its official tally of the dead to more than 28,000, though humanitarian experts say the toll could run much higher.

The warnings came as Myanmar's state media declared success in a referendum to secure public endorsement of a new constitution that critics say would perpetuate and legitimize military rule. Myanmar's leaders say the charter will lay the foundation for a "discipline-flourishing democracy."

U.N. agencies and international charities that were operating in Myanmar before the disaster have been slowly setting up operations. Emergency supplies are gradually arriving in the country and just beginning to reach the low-lying Irrawaddy Delta, but is far short of what is needed.

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