YANGON, Myanmar — Relief supplies from the United Nations arrived in Myanmar Thursday, but U.S. military planes loaded with aid were still denied access by the country’s isolationist regime five days after a devastating cyclone.

The military junta also continued to stall on visas for U.N. teams seeking entry to ensure the aid is delivered to the victims amid fears that lack of safe food and drinking water could push the death toll above 100,000.

Four airplanes carrying high-energy biscuits, medicine and other supplies arrived in Yangon Thursday, U.N. officials said. Two of four U.N. experts who had flown to Myanmar to assess the damage were turned back at Yangon’s airport for reasons that were not immediately clear, said John Holmes, the U.N. relief coordinator. The other two were allowed to enter.

By rejecting the U.S. offer, the junta is refusing to take advantage of Washington’s enormous ability to deliver aid quickly, which was evident during the 2004 Indian Ocean tsunami that killed 230,000 people in a dozen nations.

“We have demonstrated in crises around the world ... our logistical capability to get humanitarian assistance quickly in to the people who need it,” said Shari Villarosa, the top U.S. diplomat in Myanmar.

Gordon Johndroe, President Bush’s national security spokesman, said the U.S. was still working to gain permission to enter Myanmar. Another option being considered was air-dropping aid without permission, said Ky Luu, the director of the U.S. office of foreign disaster assistance.

France has argued that the U.N. has the power to intervene to help civilians because of an agreement by world leaders at a 2005 summit that the international body has a “responsibility to protect” people sometimes when nations fail to do it. But that agreement did not mention natural disasters.

Myanmar’s generals, traditionally paranoid about foreign influence, issued an appeal for international assistance after the storm struck Saturday. They have since dragged their feet on issuing visas to relief workers even as survivors faced hunger, disease and flooding.

In 2004, the first foreign military aid did not arrive in the hardest-hit nation, Indonesia, until two days after the disaster. The most significant help came when U.S. helicopters from the USS Abraham Lincoln began flying relief missions to isolated communities along the coast of Aceh province.

With roads washed out and the infrastructure in shambles, large swaths of Myanmar’s delta region also remain accessible only by air — something few other countries are equipped to handle as well as the United States.

Tim Costello, chief executive of World Vision Australia, said the U.S. has to convince Myanmar’s government that it has no political agenda.

“Clearly we all know the political context there, and I think it’s going to take a little bit more time for a breakthrough there,” Costello said.

Thailand Prime Minister Samak Sundaravej offered to negotiate on Washington’s behalf to persuade Myanmar’s government to accept U.S. aid.

“What is critically needed at this point is for Myanmar authorities to open up to a major international relief effort,” said U.N. spokesman Richard Horesy. “If that is not done quickly, there is a major risk that there will be a second phase to this disaster where large numbers of people will die of communicable disease.”

The Association of Southeast Nations appealed to the international community to keep sending aid through Thailand.