More food reached Myanmar's hungry cyclone victims as roads were cleared of fallen trees, but a British aid group warned that up to 1.5 million face death if they do not get clean water and sanitation soon.

"It's really crucial that people get access to clean water sources and sanitation to avoid unnecessary deaths and suffering," Oxfam regional chief Sarah Ireland told reporters in Bangkok, Thailand.

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She said the death toll from the May 3 cyclone could go up to 100,000 — a figure also suggested by other aid groups and a top U.S. diplomat.

"There are all the factors for a public health catastrophe which could multiply that death toll by up to 15 times," she said.

Myanmar's military junta said 23,335 people died and 37,019 were missing after Cyclone Nargis slammed into the country's Irrawaddy delta. The U.N. said about 2 million people were severely affected.

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Fishing boats along the coast helped ferry survivors to safety but diesel supplies were running low and rescuers feared time was running out for those stranded in remote delta villages.

"Some have been living on coconuts," said Maung U, a 36-year-old driver of a rescue boat in Labutta. "But even those are running out."

Yet the government has refused to let in foreign experts who have experience in handling humanitarian disasters. It insists it is capable of distributing the aid, as long as enough of it is sent by international donors.

The U.N. says some progress has been made in delivering aid.

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Boat Carrying Relief Supplies to Burma Sinks

U.N. staff in Burma —renamed Myanmar by the ruling military junta— were reporting "significant progress in clearing roadways, and the piped water supply has been partially restored to some parts of Yangon city," said an internal U.N. report seen by The Associated Press.

It said helicopter loads of international aid arriving in Yangon were being relayed to Pathein for distribution in the Irrawaddy delta.

The International Federation of Red Cross and Red Crescent Societies said three of its planes delivered 14 tons of shelter materials, replenishing its stocks already in the country before the cyclone.

"Aid is getting through in increased amounts," it said in a statement.

So far the humanitarian effort has supported 220,000 people, it said. Seven more flights were expected to arrive Monday with 20 tons of shelter materials, jerry cans and 2,000 mosquito nets.

But aid group World Vision, which has a big presence in Burma, said relief material delivered so far is a drop in the ocean.

"It is very obvious that of the thousands of people who have been helped there are tens of thousands who have not been reached," World Vision's Samson Jeyakumar said in Bangkok. He said its supplies were running out in Yangon.

Many survivors have been without help for more than a week after fleeing their inundated villages to take shelter in monasteries and schools in towns. The canals and flooded roads to higher ground were littered with the bloated bodies of humans and animals. The stench of death was everywhere.

"What is critical at the moment is water sources," said Oxfam's Ireland.

"We understand a lot of water sources are contaminated. Ponds are full of dead bodies. Something as basic as a bucket is in scarce supply. If people don't have water that is clean and safe, that is very difficult," she said.

"With floodwaters fouling water supplies and latrines overflowing with human waste, all the factors for an outbreak of cholera and shigella are in place," an Oxfam statement said. Stagnant waters can breed mosquitoes, raising fears of dengue and malaria.

Another Oxfam official, Ian Woolverton, said although the aid group has warned of a possible 1.5 million deaths, that was a worst-case forecast — and one that could be prevented.

At relief camps, long lines of people waited to collect rations of rice and oil. Where there were no camps, people clustered on roadsides hoping for handouts. "Help us!" was written in chalk on the side of one home.

"Please, don't wait too long," said Ma Thein Htwe, 49, who waited with dozens of other women and children at a monastery in Kungyangon for her ration of rice.

Ko Zaw Min, 27, said not enough aid was reaching his community. Each family was given only about a pound of rice a day.

"I want to build my home where it used to stand, in the field over there," said the farmer, who lost his 9-year-old son and month-old baby in the disaster. "But I have nothing."

Debbie Stothard, head of the Southeast Asian human rights group ALTSEAN-Burma, said the ruling generals were manipulating aid and delivering it selectively, ignoring the needy.

"Even in the Yangon area, which is reachable by the regime, people are complaining they are not getting aid. What they are getting is rotting rice," she said in Bangkok.

Burma's junta has also been criticized for holding a referendum on Saturday on a new constitution aimed at solidifying its hold on power.

The referendum seeks public approval of a new constitution which guarantees 25 percent of parliamentary seats to the military, allows the president to hand over all power to the military in a state of emergency, and bans pro-democracy leader Aung San Suu Kyi from public office — elements critics say defy the junta's professed commitment to democracy.