Teams of aid experts allowed into the cyclone-ravaged Irrawaddy delta have returned to Thailand with the bleakest of warnings: Burma is on the brink of a “devastating public health crisis” if help is not allowed to flow across its borders immediately.

An American cargo aircraft is expected to land in Burma today, carrying the only US aid the junta has allowed into the country as supplies pile up by the side of runways. For the hundreds of aid workers who have flown into Bangkok from around the world, the chances of entry visas materialising appeared to fade.

Rumours have begun circulating that the Burmese regime is preparing to close its doors altogether – a decision, UNaffiliated aid workers said, that would cost the lives of thousands.

Burma’s state television yesterday revised its count of the number of dead to 28,458 and the number of missing to 33,416, but aid workers believe that the final toll could reach 100,000. Deaths from hunger and disease could be far higher, they add.

Burma says that it will accept aid from all countries but does not want to let in any more foreign workers who would carry out and manage the operations. So far, only a few of the assessment teams that would normally be on the scene have been allowed in to do their jobs. Those now returning to Thailand paint a grim picture of what is needed.

After three days investigating the worst-affected parts of the country, assessors said that the junta’s de facto blockade on large-scale foreign assistance was in danger of creating a separate refugee crisis. Tens of thousands of people left homeless by Cyclone Nargis have, until now, stayed put in the hope that food, shelter and medical supplies were on their way. Now that more than a week has passed, many have set off in search of salvation.

Voting booths sprang up among the destruction over the weekend as the junta pushed ahead with a referendum on a new constitution.

The White House said it hoped that the C130 aircraft’s flight would be the first of many. “This is a very vulnerable population, and a shock of this magnitude is going to take people right off the cliff,” Ky Luu, of the Office of US Foreign Disaster Assistance, said. “We will be able to make a difference,” he added. “People are dying, and it’s approaching a week.”

The US military has C130 cargo aircraft and about a dozen helicopters in the region. US Navy ships have also begun moving from the Gulf of Thailand toward the country.

President Bush has said that the US wants “to do a lot more”, but added last week: “In order to do so, the military junta must allow our disaster assessment teams into the country.”

One of the teams sent by the aid group World Vision said it had discovered 30,000 refugees encamped around the village of Myaungmya. “These are desperate people who, after a week of waiting, are giving up on the aid response and heading off in search of food and shelter,” Chris Webster, one of dozens of World Vision workers trapped in Bangkok, said.

Although some medical and food supplies have begun to trickle into the country, disaster experts said that they fell far short of what was needed. Stephan Goetghebuer, of Médecins sans Frontières, said that while the Burmese authorities had allowed just enough leeway for a few aid shipments, “it’s no more than a drip-feed, really, given a serious response is more than required”.

Dysentery and diarrhoea would, said medical observers, tear through the storm’s survivors as they struggled to rebuild their lives. The warehouses of undelivered medical and food supplies are infuriating aid groups.

The lack of expertise on the ground is heightening the crisis: the junta’s stonewalling means that disaster specialists are pacing their offices in Bangkok just a few hundred miles from where they should be.

They are not the only ones waiting in Thailand in dread over the situation. Khin, who works as a gardener in Bangkok, is one of the hundreds of thousands of Burmese people living in Thailand with family back home – family he now believes have been claimed by the cyclone. Over the weekend, Burmese migrants protested outside their own embassy in Bangkok, demanding that the regime drop its objection to foreign aid.

Holding a newspaper bearing aerial photographs of the devastation, Khin wept as he thought of his sister and her family. “It is terrible not to know, but it is worst to think that they might have been saved if the foreign aid had been allowed in,” he said.

Among aid workers who have flown into Bangkok from around the world, emotions have swung wildly between hope and despair as the odd visa application has been processed only for many others to sink without a trace.

Donations to the British Disaster Emergency Committee can be made by calling 0870 6060900

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