Many Rohingya Muslims are feared drowned after a boat carrying of them out of danger from a cyclone capsized off western Myanmar.
It was the start of a mass evacuation from low-lying regions ahead of cyclone Mahasen.
Barbara Manzi, head of the Myanmar office of the UN Office for the Coordination of Humanitarian Affairs, said the boat struck rocks off Pauktaw township, in Rakhine state, and sank.
The United Nations warned last week that the tropical cyclone could bring "life-threatening conditions" to thousands of people living in camps in the west of Myanmar.
They fled after their homes were destroyed in violence between majority Buddhists and minority Muslims last year.
"The possibility of the cyclone hitting has added a real urgency to our plans," said Vivian Tan, a spokeswoman for the UN refugee agency.
"We're building temporary shelters made out of bamboo which will hopefully stand the heavy rains, but with a cyclone, you just don't know what will hold up."
Evacuations began on Monday in some areas such as Pauktaw, a port town about 27 kilometres by sea from the Rakhine state capital of Sittwe.
Nearly 20,000 Rohingyas are living in makeshift camps in Pauktaw after last year's violence.
This includes about 12,000 in flood-prone areas, according to UN data.
The mostly stateless Rohingya Muslims are denied citizenship by the Myanmar government and considered by many in the country to be immigrants from Bangladesh.
The slow-moving cyclone Mahasen was raised to a category four warning level by Bangladesh authorities on Monday.
© ABC 2013
15:10 EST Hot dry and gusty northwesterly winds ahead of a cooler change are causing severe fire danger over South Australia's West Coast, Eastern Eyre and Lower Eyre Peninsulas today.