The death toll from a killer cyclone in Myanmar could be "in the region of 100,000 or even more," the chief of the United Nations Humanitarian Affairs said Wednesday, according to CNN.

John Holmes, U.N. under-secretary general for humanitarian affairs, said his organization already has confirmed 38,491 deaths -- much more than the 22,000 figure from Myanmar`s government.

The weather forecast, meanwhile, calls for rain in the next several days in Myanmar, which is also known as Burma. That could cause flooding in low-lying areas that already are saturated with water left when Cyclone Nargis slammed into the country on May 2.

In another development Thursday, Myanmar`s government announced that a military-backed constitution was overwhelmingly approved by voters in last week`s referendum, The Associated Press reported.

State radio said Thursday that the draft constitution was approved by more than 92 percent of the 22 million eligible voters. It put turnout at more than 99 percent, AP reported. Human rights groups have denounced the referendum as a way for the junta to solidify military rule.

Aid agencies have struggled to gain access to the country from the secretive military junta that rules Myanmar, though some relief flights have arrived this week. The regime has indicated that it would like supplies but not international aid workers.

That lack of access makes it hard to bring the scale of destruction into sharp focus.

Citing figures from 22 organizations in 58 townships, the International Federation of Red Cross and Red Crescent Societies said estimates of the death toll in Myanmar range from 68,833 and 127,990.

"They are all estimates, which may or may not be right," said spokesman John Sparrow. "There is no way to verify the figures, no way any organization could substantiate them."

The International Federation of Red Cross and Red Crescent Societies does not formally estimate death tolls, he said. It compiled figures through an informal survey of numbers cited by other organizations. Those groups say the cyclone affected from 1.6 million and 2.5 million people.

Forecasts show that within a week, the Irawaddy Delta -- the part of the country hardest-hit by the cyclone -- could receive another 12 cm (4.7 inches) of rain.

"Clearly there`s a huge frustration that while (aid workers) may be able to get into the country and into Yangon, they`re not at the moment able to move into the affected areas and carry out the tasks they normally carry out," Holmes said.

In recent days, Myanmar has agreed to let in some foreign aid.

Thailand`s Prime Minister Samak Sundaravej, back from a visit to Yangon, said the military junta had guaranteed him that there were no disease outbreaks or starvation among the survivors.

He said Myanmar did not want any foreign aid workers because they "have their own team to cope with the situation."  Watch aid being unloaded in Yangon »

A U.S. Marine spokesman told CNN that the government had authorized five more U.S. flights to land in Myanmar with supplies. The flights will deliver 46 pallets loaded with bottled water, plastic sheeting and hygiene kits as well as crackers and powdered milk.

Three additional U.S. flights have already gone to Myanmar -- one on Monday and two on Tuesday. They carried food, mosquito netting and tarpaulins. Watch what Myanmar`s children are going through

Meanwhile, Pentagon officials said the USS Essex, USS Juneau and USS Harpers Ferry are in international waters off the coast of the country also known as Burma, laden with more than 14,000 containers of fresh water and other aid and awaiting orders to deliver by air or landing craft.

On Wednesday, British Prime Minister Gordon Brown asked U.N. Secretary-General Ban Ki-moon to convene an emergency summit on Myanmar aid.

Ban has blasted the reclusive regime for what he called an "unacceptably slow response" to the disaster, and called, "in the most strenuous terms, on the government of Myanmar to put its people`s lives first."