The UN's top disaster official headed Sunday to Myanmar, where the government is under mounting pressure to accept a full-scale relief operation for desperate cyclone survivors in need of immediate aid.

The secretive military rulers have let more foreign experts into the country in recent days to help the estimated two million survivors who do not have enough food, water or shelter more than two weeks after the storm struck.

But with emergency relief coordinator John Holmes due to arrive late Sunday, a UN report said needs were still critical, while British aid group Save the Children said thousands of children could starve to death within weeks.

"We are extremely worried that many children in the affected areas are now suffering from severe acute malnourishment, the most serious level of hunger," said Jasmine Whitbread, chief executive of Save the Children UK.

"When people reach this stage, they can die in a matter of days. Children may already be dying as a result of a lack of food."

Despite thousands of tonnes of aid being flown in, relief groups want fuller access to help supervise relief in the aftermath of the May 2-3 storm, which the government says left nearly 134,000 people dead or missing.

"Unofficial figures are considerably higher," the internal UN report said. "Food, shelter, medical supplies and water remain critical needs."

The international community has been toughening the rhetoric on the country's military rulers, who are deeply suspicious of the outside world and have limited access to foreigners with expertise in managing disaster zones.

South Africa's Archbishop Desmond Tutu and French Foreign Minister Bernard Kouchner have both raised the spectre of crimes against humanity by the junta over its handling of the catastrophe.

Despite the government's insistence that the relief effort is going well, witnesses who managed to sneak through the security cordon around the hard-hit Irrawaddy Delta said the situation remained dire.

"It was horrible beyond description," said a foreign businessman, one of about a dozen eyewitnesses interviewed by AFP.

"Most of the devastated huts looked like they were empty at first glance. But there were actually survivors inside," he said.

"One hut with no roof was full of about 100 people, crouching in the rain. There was no food and no water. Each person had nothing more than the clothes on their bodies, shivering in the cold."

The junta has continued to insist it can handle most of the relief operation by itself, and state media are full of photos of smiling and grateful citizens receiving aid supplies from generals.

The junta's English-language newspaper, the New Light of Myanmar, on Sunday carried more than two dozen stories praising the regime's relief efforts.

Aid agencies are hoping that Holmes will have some sway on the generals, who keep an iron grip one of the poorest and most isolated nations on the planet.

Amanda Pitt, regional spokeswoman for the UN's disaster relief arm headed by Holmes, said he would go straight from New York to Myanmar's main city Yangon, where he would spend "a few days" trying to get a picture of the situation.

She said UN Secretary General Ban Ki-Moon had "asked him to travel to the region to better assist the survivors and help the government of Myanmar scale up the response to the crisis."

She had no information on which areas Holmes would be able to visit, or if he would have access to the top junta leadership.

Wary of any foreign influence that could weaken its 46 years of iron rule in Myanmar, the military has insisted on managing the relief operation itself and kept most international disaster experts away.

But aid groups say the government cannot possibly handle the tragedy alone, with hundreds of tonnes of supplies and high-tech equipment piling up in warehouses, bottle-necked by logistics and other problems.

As pressure mounts on Myanmar's allies to use their clout with the regime, Southeast Asian foreign ministers will meet in Singapore on Monday for talks on how to deal with their neighbour.