At least 50 Rohingya Muslims are feared drowned after their boats capsized off western Burma, officials say.
The boats, said to be carrying about 100 passengers, were evacuating people ahead of powerful Cyclone Mahasen.
The vessels sank off Pauktaw township in Rakhine state late on Monday. More than 40 survivors have been found and eight bodies recovered.
Thousands of Rohingya Muslims are living in temporary camps in Rakhine after violence last year.
The UN had called for an urgent evacuation ahead of the storm, warning that many areas where displaced people are now living are in low-lying coastal areas at risk of flooding or tidal surges.
Aid agencies said that the three boats got into trouble after setting out on Monday night.
Only one boat in the convoy had an engine, towing the other two smaller vessels. Reports say the vessels were overcrowded.
Local resident U Than Htun said the boat was crossing the mouth of the Sittwe River on its way to Sittwe, the state capital some 27km (17 miles) away by sea, when it sank.
"There is a refugee camp nearby and when the security heard the news, some soldiers rushed to rescue 30 people," he told the BBC Burmese service.
"One Bengali lady who came ashore was also saved by Thekone villagers and informed the authorities. The villagers also saw about four persons who landed on an island where there is an old light house."
Barbara Manzi, head of the United Nations Office for the Co-ordination of Humanitarian Affairs (UNOCHA), told the BBC from Sittwe it appeared the boats "left the camp with the blessing of the authorities before hitting rocks".
Earlier reports suggested that up 200 people were on board, but the UN later revised the number to about 100.
Burmese officials began evacuations this week, after warnings the cyclone might hit neighbouring Bangladesh from Thursday, bringing heavy rain and flooding to western Burma.
This could hit an estimated 140,000 displaced people - mostly Rohingya - who are living in makeshift shelters in Rakhine, aid groups say.
They have been displaced since violent clashes between Rakhine's Muslim and Buddhist communities in June and October 2012.
"The government has been repeatedly warned to make appropriate arrangements for those displaced in Rakhine state," Isabelle Arradon, deputy Asia Pacific director of the rights group Amnesty International, said in a statement on Monday.
"Now thousands of lives are at stake unless targeted action is taken immediately to assist those most at risk."
Brad Adams, Asia director at Human Rights Watch, said that if the government failed to evacuate those at risk, "any disaster that results will not be natural but man-made".
But some people have reportedly refused to leave because they fear having nowhere else to go.
"We are very worried about the cyclone... we do not have enough food to eat," a member of the Rohingya community told Agence-France Presse news agency.
"Many people are in trouble. But we have no idea what we should do."
According to Nasa, Cyclone Mahasen was north-east of Sri Lanka on Monday. It was expected to strengthen as it moved north and hit Burma late in the week, the agency said.
Five years ago, Cyclone Nargis struck Burma's Irrawaddy Delta region, killing at least 140,000 people and leaving three million in urgent need of assistance.