The United Nations bluntly warned the Myanmar junta on Monday that more lives will be lost unless the aid effort to reach survivors of a devastating cyclone picks up speed immediately.

Ten days after Cyclone Nargis devastated the country's southern delta, washing away whole villages and leaving 62,000 dead or missing, UN relief officials said bottlenecks and delays had to be resolved now.

They cautioned that an increase in aid flights was not sufficient if the regime did not issue visas for foreign disaster experts and get help to the neediest victims in one of the world's poorest and most isolated nations.

The relief operation was only at 10 percent of the level needed to bring water, food and supplies to desperate survivors, and that just 20 percent of the food required was making its way in, they said.

"I would urge that we don't judge the success of this operation by flights arriving alone," Richard Horsey, a spokesman for the UN's emergency relief arm, said in neighbouring Thailand.

"This is a huge disaster," Horsey said in an interview with AFP TV. "It would overwhelm the capacity of any country."

The first aid flight from the United States since the cyclone hit overnight on May 2 arrived in Yangon on Monday, and the trickle of flights from the outside world has been slowly increasing.

But up to two million people are in need of immediate relief supplies. Many in the battered southern Irrawaddy Delta are without food or fresh water, and there are reports of widespread outbreaks of diarrhoea and dysentery.

"Access is a key question," said Terje Skavdal, of the UN's emergency relief arm.

"There are limitations if not bans on international staff going into the delta.

"Our national staff are doing a great job, but in this situation we also need to get the right experts and in the right numbers."

Relief officials have repeatedly said that specialists in disaster zones are essential to oversee the aid operation on the ground but the regime, deeply suspicious of the outside world, will not let them in.

Dozens of aid workers are awaiting visas, according to United Nations figures.

"Faster progress on this issue is crucial to the effectiveness of the response," Catherine Bragg, deputy head of the UN's emergency relief arm, said in a statement.

"If we do not act now, and we do not act fast, more lives will be lost."

The warnings came as Myanmar's military government admitted that it had still been unable to reach all the affected areas of the country on the ground, and that in some places it was simply dropping supplies from helicopters.

The United Nations issued an emergency appeal for 187 million dollars to help cope with the tragedy, and issued stark new statistics about the situation on the ground.

In Yangon, the rice warehouse of the UN's food agency was empty. In the disaster zones, an estimated 20 percent of children are suffering from diarrhea due to a lack of sanitation and clean water.

The United Nations said 50 million dollars was needed just to help with logistics alone, including building storage infrastructure to keep donated food supplies from spoiling before they reached the hungry.

"I strongly urge the government to reconsider its attitude," Bragg said. "The immense challenge of getting the aid onwards to the people in such desperate need in the delta area cannot be overstated."

BYLINE: Claire Truscott , Agence France Presse