YANGON, Myanmar  The directors of several relief organizations in Myanmar said Wednesday that some of the international aid arriving into the country for the victims of Cyclone Nargis was being stolen, diverted or warehoused by the country’s army.

The United States military’s Joint Typhoon Warning Center said there was a possibility that “a significant tropical cyclone”  a second big storm  would form within 24 hours and head across the Irrawaddy Delta, the region that suffered most from the first storm, which struck May 3.

Thailand’s prime minister, Samak Sundaravej, flew to Yangon on Wednesday to try to persuade Myanmar’s leaders to allow more foreign aid workers into the country. The members of the military junta told him they were in control of the relief operations and had no need for foreign experts, he told reporters after returning to Bangkok, The Associated Press reported.

The government said there were no outbreaks of disease or starvation among the hundreds of thousands of people affected by the cyclone. In Yangon, Mr. Samak met with the prime minister, Lt. Gen. Thein Sein, The A.P. report said.