STORY: Cyclone Mocha, packing winds of up to 210 kph (130 mph), could bring sea surges of up to 12 feet (4 meter) affecting more than 2 million people directly in its path, most of them in Myanmar's Rakhine and Chin states.
Many more could suffer as the storm moves inland from the Bay of Bengal, the United Nations Satellite Center has warned.
It's forced hundreds of thousands of people to evacuate from their homes, many of them frail structures in low-lying areas.
At least 100,000 people in Myanmar's impoverished Rakhine state have moved to safer areas since last week, said a major ethnic militia and the UN's humanitarian office (OCHA).
About 6 million people are already in need of humanitarian assistance and 1.2 million are displaced in Rakhine and the northwest, OCHA says.
Myanmar has been plunged into chaos since a junta seized power two years ago. After a bloody crackdown on protests, a resistance movement is fighting the military on various fronts.