When Tropical Cyclone Nargis struck Myanmar, the winds ripped off many of the zinc roofs at the Insein prison outside Yangon, the Thailand-based Assistance Association for Political Prisoners said in a statement.

Guards forced about 1,000 prisoners to weather the storm inside a prison hall. When some inmates started a fire to keep warm, others panicked and a riot broke out, the group said.

Soldiers and riot police were called in to quell the riot, and opened fire on the prisoners, it said. The clash left 36 prisoners dead and 70 injured, the group added.

"It is a merciless and unnecessary action to shoot. It is brutal and we would like to have independent investigation on that matter," said Tate Naing, the group's secretary.

State television said Monday that nearly 4,000 people were killed in the cyclone that tore into the impoverished and secretive Asian nation at the weekend, and that tens of thousands more could also be dead.

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