As misery grew for a sixth day for uncounted survivors of the devastating cyclone in Myanmar, the United Nations said Friday that the government had seized its relief supplies in Yangon, while a Pentagon official said that the junta had come to a breakthrough agreement to allow a single American aid plane to land on its territory.

“We hope that this is the beginning of broader support between the United States and Burma to help the Burmese people,” said the Pentagon official, speaking on condition of anonymity to discuss diplomatic developments.

Both developments underscored the painstaking negotiations over aid and its excruciatingly slow pace as the country’s iron-fisted military rulers block relief workers and most supplies. Altogether, by one count, 11 chartered planes with relief supplies have landed in Myanmar, a tiny amount for a disaster that the United Nations said may have affected 1.5 million people

Six days after the cyclone struck, ravaging much of Myanmar’s coast, the scale of the damage has yet to be fully assessed.

Reports from inside the country, including from the few outside organizations allowed to operate there, suggest that food and clean water are scarce or entirely unavailable in the worst-struck areas, and that the death toll could escalate from the official count of 22,500. Aid officials and health experts are deeply worried about new ravages from diseases like cholera and acute diarrhea. But there is little hard data.

The first United Nations flights arrived Thursday, carrying high energy biscuits, water containers, food and plastic sheets. But one aircraft and two relief workers were reported to have been turned back at the airport.

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And on Friday, the United Nations World Food Program said the aid it had delivered had been seized. “All the food aid and equipment that we managed to get in has been confiscated,” said Paul Risley, a spokesman for the United Nations World Food Program in Bangkok.

After saying it would halt deliveries, the agency said flights would continue on Saturday while the issue is worked out.Secretary General Ban Ki-moon urged the Myanmar authorities to let aid into the country “without hindrance” and said the effect of further delay could be “truly catastrophic.”

His spokeswoman, Marie Okabe, said Mr. Ban had been trying for two days without success to get in touch by telephone with Than Shwe, the regime’s senior general. “We have been told that the phone lines are down,” she said.

Myanmar’s military junta said in a statement on Friday that it was willing to receive disaster relief from the outside world but would distribute supplies itself rather than allowing in relief workers. But aid agencies want to coordinate and control their own aid.

Already Myanmar has turned away one fully loaded flight becauese the supplies were accompanied by disaster exeperts and press.

“Myanmar is not in a position to receive rescue and information teams from foreign countries at the moment,” said a Foreign Ministry statement.

“But at present Myanmar is giving priority to receiving relief aid and distributing them to the storm-hit regions with its own resources.”

But some agencies and nations were delivering supplies successfully.

India sent two ships loaded with relief supplies and the United Nations Children’s Fund said it was not meeting problems with its deliveries of aid.

A spokesman for Unicef, Christopher de Bono, said in an e-mail that millions of water purification tablets had been delivered thursday and that although customs clearance could take two days, “as far as we know there has been no indication of any problems so far.”

In a telephone call from Myanmar, an official of the International Red Cross, Micheal Annear, said delivery work was proceeding normally in cooperation with other agencies and local businesses.

Doctors Without Borders, which had been running large H.I.V. and malaria programs in Myanmar, has about 80 staff members in the Delta region and is sending more in, said Frank Smithuis, the group’s head of mission. In a telephone interview from Yangon on Friday, he said the group was distributing food and medicine from the stores it already had in place.

In the worst-affected areas, he said, 95 percent of the people had lost their homes and everything they owned, and were in desperate need of food, water and shelter.

Mr. Smithuis said his group was dispatching teams of six — a doctor, a nurse, a medical assistant, two water and sanitation workers and a food distributor who would hire local people to help distribute food.

The teams are seeing many people injured by the storm who have infected wounds that need to be drained and treated with antibiotics, he said.

“It sounds like we have everything under control and that’s not true,” Mr. Smithuis said. “The area is wide and there’s a lot of people. We don’t see other players, we don’t see other help.” Most relief workers on the ground are local people and would be less likely to encounter the suspicion with which authorities view foreigners.

Mr. Risley of the World Food Program said he had never seen delays like this before. In Indonesia after the tsunami in 2004, he said, an air bridge of daily flights was established within 48 hours.

“The frustration caused by what appears to be a paperwork delay is unprecedented in modern humanitarian relief efforts,” he said. “It’s astonishing.”

He said his agency alone had submitted 10 visa applications for relief workers but that none had been approved before consulates shut down for the weekend.

“We strongly urge the government of Myanmar to process these visa applications as quickly as possible, including working over the weekend,” he said.

Bettina Luescher, a spokeswoman for the World Food Program, turned aside repeated questions at the United Nations of what led her agency to make its original decision to suspend relief and then rapidly reverse it.

“All I can say is that at our headquarters in Rome, there were discussions going on and it was decided that we should send in those planes tomorrow,” she said.

She said that of the 16 visas for entry into Myanmar sought for international staff members, only one had been granted, and it had been requested prior to the cyclone.

The White House welcomed the news on Friday that Myanmar would allow some American aid in on Monday. And the Pentagon official said: “We will seek to work with our partners who can get the supplies to the people who need them. We hope that this is the beginning of broader support between the United States and Burma to help the Burmese people.”

Among the forces the United States could call on is the Essex Strike Group, which was in the region for Cobra Gold military exercises with Thailand. The group transferred a dozen transport helicopters to Thailand, where they could fly to Myanmar in a matter of hours with relief supplies. The ships are moving toward waters off Myanmar to be available with medical and other relief or reconstruction capabilities on board.

Aboard the ships are amphibious landing craft that can move onto battered shorelines and can carry personnel and supplies to remote locations, inaccessible by road.

“We will come, provide assistance, and then leave — just like in Bangladesh, Indonesia, and other places where we have provided assistance,” said Maj. Stewart Upton of the Marine Corps, a Pentagon spokesman.

Military officials said a number of ground troops had been pulled from the 1,800-strong 31st Marine Expeditionary Unit aboard the Essex Strike Group, and had moved into Thailand.

The Cobra Gold Exercise, which remains under way, had missions that included the deployment of heavy moving equipment that could be used in clearing and reconstruction in the cyclone zone, officials said.

Among the ships, the Essex can produce 50,000 gallons of fresh water per day, while the Harpers Ferry and Juneau each can produce 10,000 gallons of fresh water per day.

Other navy ships in the region include the Blue Ridge, serving as 7th Fleet flag ship; the Kitty Hawk carrier strike group; the carrier Nimitz, and the destroyer Mustin. All have standard hospital and relief capabilities on board.

Four KC-130J transport and fuel planes have landed in Korat, Thailand, and three C-130 transports now are in Utaphao, Thailand, officials said. Also, a larger C-17 was flown to Thailand with water purification systems and packaged military meals on board.

A team of four dozen military personnel also flew to Thailand, officials said. The team is trained to set up an airstrip in difficult terrain so that relief aircraft can land. But they are waiting.

Warren Hoge contributed reporting from the United Nations, Thom Shanker from Washington and Denise Grady from New York.