BANGKOK, Thailand — Two U.N. transport planes loaded with cyclone relief supplies landed in Burma yesterday, as international leaders heightened pressure on the country’s secretive military government to fully embrace foreign help. U.N. Secretary-General Ban tried unsuccessfully to telephone Burma’s top general to press the case personally.

U.S. military transports were standing by in Thailand to fly in more supplies. But no clearance arrived from Burma, where U.N. experts now estimate that 1.5 million people are in need of help after last weekend’s cyclone.

“Burma has got to open itself up to a major international effort very soon if we are not to face a second disaster, where infectious disease and other problems start to take a significant toll,” a spokesman for the U.N. Office for Coordination of Humanitarian Affairs, Richard Horsey, said Richard Horsey.

Despite hunger, diarrhea outbreaks, and general desperation, Burma’s military authorities continue to resist the kind of massive international relief that followed the 2004 Indian Ocean tsunami and the Pakistan earthquake of 2005. The leaders are highly suspicious of American and other Western governments, which have condemned them as dictators.

The American ambassador to Thailand, Eric John, expressed chagrin at the continuing stalemate, saying sluggish bureaucracy could be partly to blame. “It is very frustrating, if you look at the people’s suffering,” he said. “You have the tools at your fingertips to alleviate that suffering, and they are just not picking them up.”

Mr. John said America thought it had received a green light yesterday morning to send in C-130 military transport aircraft carrying relief supplies. But Burma authorities later made it clear no permission had been granted.

In New York, U.N. officials said that Mr. Ban had tried to call the head of Burma’s military and government, Senior General Than Shwe, but that repeated attempts yesterday failed. Mr. Ban also urged postponement of a constitutional referendum across the country so as to focus all resources on the disaster. This week, the government said it would delay the vote scheduled for Saturday by two weeks in the cyclone zone but proceed with it as scheduled in other parts of the country.

The chief U.N. emergency relief coordinator, John Holmes, said only two World Food Program officials had been issued visas to enter the country, leaving more than 40 U.N. relief workers stranded in Bangkok