A powerful cyclone killed at least 351 people and destroyed thousands of homes, state-run media said Sunday.
The storm's 120-mph winds blew the roofs off hospitals and cut electricity to the country's largest city.
Shari Villarosa, the top American diplomat in Yangon, said the whipping winds and torrential downpour had caused "major devastation."
The dead included 162 people on Haing Gyi island off the southwest coast, the military-run Myaddy television station reported. Many of the others died in the Irrawaddy delta.
"The Irrawaddy delta was hit extremely hard not only because of the wind and rain but because of the storm surge," said Chris Kaye, the U.N.'s acting humanitarian coordinator in Yangon, also known as Rangoon. "The villages there have reportedly been completely flattened."
State television reported that in the Irrawaddy's Labutta township, 75% of the buildings had collapsed Saturday.
Some dissident groups worried that the armed forces running Myanmar -- also known as Burma -- would be reluctant to ask for international help.