Two waves of deadly communal violence between Buddhists and Muslims in Rakhine in 2012 left some 200 people dead and around 140,000 displaced, mainly Rohingya.
"Two years after inter-communal violence erupted in Myanmar's Rakhine state, thousands of people are still leaving by boat from the Bay of Bengal," UNHCR spokesman Adrian Edwards told reporters in Geneva.
"Reports of abuse and exploitation as people seek safety and stability elsewhere are meanwhile increasing," he said.
The UNHCR estimates that more than 86,000 people have left the area by boat from the Bay of Bengal since June 2012, including 15,000 between January and April this year alone.
More From This Section
"People who have made it to Thailand, Malaysia or Indonesia have told UNHCR staff about overcrowded boats that sometimes lost their way or developed engine problems," Edwards said.
Boats sometimes ran out of food and water, and people who died along the way were said to have been thrown overboard, he said.
"They recount daily beatings and that some people died," he said, adding that "they spent their days sitting in confined spaces and nights sleeping upright or in foetal position due to the lack of space".
The stateless Rohingya are considered by the UN to be one of the world's most persecuted minorities.
Many of the displaced have gone without access to healthcare and other help after attacks on international humanitarian groups by Buddhist extremists earlier this year.