Desperate survivors of Cyclone Nargis headed out of Myanmar's Irrawaddy delta in search of food, water and medicine, but aid workers said on Sunday that thousands will die if emergency supplies don't get through soon.

Buddhist temples and schools on the outskirts of the storm's trail of destruction are now makeshift refugee centres.

The UN humanitarian agency said in a new assessment that between 1.2 million and 1.9 million were struggling to survive in the aftermath of the storm that struck eight days ago.

"Given the gravity of the situation including the lack of food and water, some partners have reported fears for security, and violent behaviour in the most severely afflicted areas," the United Nations Office for Coordination of Humanitarian Affairs said.

It said "the number of deaths could range from 63,290 to 101,682, and 220,000 people are reported to be missing". It said "acute environmental issues" posed a threat to life and health.

In 1991 a cyclone slammed into neighbouring Bangladesh, killing 143,000 people.

While Myanmar's reclusive military government is accepting aid from the outside world, including the United Nations, it will not let in the foreign logistics teams.

"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 Greg Beck of the International Rescue Committee.

In the delta town of Labutta, where 80 percent of homes were destroyed, authorities were providing one cup of rice per family per day, a European Commission aid official told Reuters.

In a blow to the stumbling relief effort a boat carrying some of the first aid to survivors sank, the International Federation of Red Cross and Red Crescent Societies said.

The boat was believed to have hit a submerged tree in the Irrawaddy delta. The accident highlighted the enormous logistical difficulties of delivering aid, with roads washed away and much of the delta turned to swamp.

Myanmar raised the death toll on Sunday to 28,458 dead and 33,416 missing from the storm on the night of May 2 and early on May 3. Most of the victims were killed by the 12-foot (3.5 metre) wall of sea-water that hit the delta along with the Category 4 cyclone's 190 kph (120 mph) winds.

International agency Oxfam said 1.5 million people are at risk from disease unless a tsunami-like aid effort is mobilised.

"In the Boxing Day tsunami 250,000 people lost their lives in the first few hours, but we did not see an outbreak of disease because the host governments and the world mobilised a massive aid effort to prevent it from happening," Oxfam's Regional Director for East Asia Sarah Ireland said in Bangkok.

"We have to do the same for the people of Myanmar."    

The cyclone is one of the worst disasters since the December 26, 2004 tsunami that hit a dozen countries along the Indian Ocean.

Australia responded to a UN appeal for $187 million in aid by dramatically increasing its contribution to $23.4 million.

The UN World Food Programme said on Sunday it has begun moving aid to its field headquarters in Labutta using trucks provided by its partners in Myanmar, including the Myanmar Red Cross. The agency said its food shipments had been briefly impounded on Friday at Yangon airport.

France is set to deliver 1,500 tonnes of rice aid aboard the warship Mistral, which would arrive in Myanmar's waters in the middle of this week, the French foreign ministry said on Sunday.

France wants the aid on the Mistral to be distributed either by the ship's crew, or by the staff of NGOs already on the ground, or by UN teams, a foreign ministry source said.

French Foreign Minister Bernard Kouchner told French newspaper Le Figaro on Saturday that France would not consider entrusting aid to the Myanmar authorities.