Relentless monsoon rains have triggered flash floods and landslides, destroying thousands of houses, farmland, bridges and roads - with fast-flowing waters hampering relief efforts.
Hundreds have also perished in recent days in India, Nepal, Pakistan and Vietnam following floods and landslides triggered by heavy seasonal rains.
In Myanmar, "46 people have died and more than 200,000 have been affected by the floods across the country", an official at the Relief and Resettlement Department told AFP.
Myanmar is a vast and poor country, where communications and infrastructure are already weak, prompting the United Nations to warn that a full picture of the scale of the disaster may not emerge for days.
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Authorities have declared the four worst-hit areas in central and western Myanmar "national disaster-affected regions".
In the impoverished northern Sagaing Region, residents said the flood waters caught them off guard as they swept into villages, swamping homes and fields.
"There was no warning... We thought it was normal (seasonal flooding)," Aye Myat Su, 30, told AFP from a monastery being used as a temporary shelter in the regional capital of Kalay.
Landslides in Chin State -- south of Sagaing -- have destroyed 700 homes in the state capital Haka, while more than 5,000 people in another district are in relief camps, the state-backed Global New Light of Myanmar reported today.
President Thein Sein has promised the government will do its "utmost" to provide relief, but said parts of Chin had been "cut off from surrounding areas", the report added.
Myanmar's Health Ministry says it is distributing medical supplies across the country including chlorine tablets, although it was unclear how it will reach many of the afflicted zones, with boats and helicopters in short supply.