The crew managed to get to the safety of an island, along with four Red Cross staff who were on the boat," Red Cross official Joe Lowry said. "But we've lost most of the cargo."

The double-decker relief boat was carrying about 100 bags of rice, 5,000 liters of drinking water, 10,000 water purification tablets, 200 water storage containers, 10 stretchers, 30 boxes of clothing and household items like soap.

A submerged tree damaged the hull of the boat as it headed toward the delta township of Bogalay, which was hit hard by the cyclone, Lowry said. The boat sank quickly, leaving its cargo at the bottom of a muddy river, he said.

The boat was traveling from Yangon to Mawlamyinegyun when it sank, according to the Red Cross.

A US military transport aircraft with relief supplies is expected to land in Myanmar on May 12 on a mission that officials hope will help forge a relationship to allow the United States to send in disaster experts.

The United Nations estimates the death toll from the May 2 Cyclone Nargis ranges from 63,000 to 100,000, well above the Myanmar government's estimate of about 22,000. Tens of thousands of people are missing.