YANGON, MYANMAR - A U.S. plane ferried relief to Myanmar for the first time Monday to help nearly 2 million cyclone victims facing disease and starvation, but the U.N. chief criticized the military junta for its "unacceptably slow response."

Even as the death toll climbed, Myanmar's authoritarian regime continued to bar nearly all foreigners experienced in managing humanitarian crises from reaching survivors of Cyclone Nargis.

With hundreds of thousands of homes destroyed in the disaster zone, refugees packed into Buddhist monasteries or camped in the open, drinking dirty water contaminated by dead bodies and animal carcasses. Medicine and food were sorely lacking -- even as supplies piled up at the main international airport.

Yangon, Myanmar's largest city, was pounded by heavy rain Monday, and more downpours were expected throughout the week, further hindering aid deliveries. For many, the rainwater was the only source of clean drinking water.

U.N. Secretary-General Ban Ki-moon chided the junta for its "unacceptably slow response" in helping victims of the disaster and warned of a deepening crisis.

"Unless more aid gets into the country -- very quickly -- we face an outbreak of infectious diseases that could dwarf today's crisis," he said. "I therefore call, in the most strenuous terms, on the government of Myanmar to put its people's lives first."

Myanmar's regime made a huge concession Monday by letting the United States -- the fiercest critic of its human rights record -- bring in relief following prolonged negotiations.

The U.S. military C-130 cargo plane filled with 14 tons of water, mosquito nets and blankets was unloaded in Yangon, providing help for 30,000 victims of the May 3 disaster.

U.S. military officials said they hoped it would be the start of a steady flow of aid, with two more flights planned for today.

"We hope they will allow us to do more in the future," said Lt. Col. Douglas Powell, the U.S. Marines spokesman for the operation. "It's really just up to what the Burmese will allow us to do."

Adm. Timothy J. Keating, the commander of the U.S. military in the Pacific, was on board Monday's relief flight to try to negotiate with the junta for a larger U.S. role.

The official death toll from the cyclone rose by nearly 3,500 Monday to 31,938, with another 30,000 missing. The United Nations and others have said the death toll could reach 100,000 or higher.

The first British aid flight, packed with plastic sheets to provide shelter to more than 9,000 families, was also on its way.

"The lives of thousands of cyclone survivors are at extreme risk," the World Vision aid group said. "Displaced people are living in appalling conditions in makeshift shelters and camps, where overcrowding and unsanitary conditions are prevalent."

Children -- many of them orphans -- are suffering from fever, diarrhea and respiratory infections, it said. Many survivors complained of getting rotting rice while soldiers kept the best food for themselves.

Two planes carrying 56 tons of medical and other aid from Europe-based humanitarian groups also arrived in Yangon on Monday. Three more planes were en route, Doctors Without Borders said.

Not equipped to cope

Myanmar's government has fewer than 40 helicopters, most of them old and in disrepair, and 15 transport planes, primarily small jets unable to carry hundreds of tons of supplies.

"The authorities of the country need to open up to an international relief effort," said Richard Horsey, a spokesman for U.N. humanitarian operations, in Bangkok, Thailand.

"There aren't enough boats, trucks, helicopters in the country to run the relief effort of the scale we need. It's urgent that the authorities open themselves up."

Calls mounted, meanwhile, for airdropping aid into the country, with or without the junta's approval.

"The sands of time are running out," said Britain's opposition Conservative Party leader David Cameron.

"In the end, what matters is getting aid through to people and feeding them and stopping them from dying," he told BBC Radio.

Some experts said airdrops were unlikely.

"Well, I don't think anybody now at this stage is seriously considering airdropping," said Terje Skavdal, head of the U.N. Office for the Coordination of Humanitarian Affairs. "I think the issue now is trying to build the best possible relationship with the government to get the best possible access."