The delivery of relief supplies to cyclone-ravaged Burma has picked up pace but aid groups say supplies are still falling well short of what's needed.

Our correspondent in Bangkok, Peter Cave, reports more than half a dozen giant cargo planes and several lorries have made it to Rangoon now and the first US military plane is expected to arrive late Monday.

Richard Bridle, the Deputy Regional Director of the UNICEF East Asia and Pacific Regional Office in Bangkok, has told Radio Australia's Connect Asia program the supplies are now arriving, but it remains difficult to get them to affected areas.

"From a supply point of view, we're not looking at that bad a picture - it's really just a question of a fairly slow progression of distribution of of those supplies," he said.

"We've been able to reach so far about half of the affected townships. For the others its a bit of a logistical problem because they're down in the mouth of the Irriwaddy delta."

The junta has refused to let in many foreign experts who specialise in getting aid to disaster victims, and says that only the government would be allowed to distribute emergency supplies.

The United Nations says more than half of those in desperate need, estimated by some at 1.5 million people, have seen no help at all.

The World Health Organization is also raising concerns over potential outbreaks of communicable diseases in cyclone-affected areas.

Mr Bridle says while outbreaks haven't been reported yet, the danger remains high.

"I think we're still in the stage of clear and present danger," he said.

"In these conditions, we can expect that there would probably be outbreaks of diarrhoeal disease, there might be an increase in malaria - the other disease we have to be concerned about is dengue fever, which is very contagious and can be very lethal to children."

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