Yangon: About 13,000 boat people, including many stateless Rohingya Muslims, fled Myanmar and neighbouring Bangladesh in 2012 with hundreds dying during the perilous sea voyage, the UN said on Friday.
A wave of deadly sectarian violence in Myanmar's western state of Rakhine has triggered an exodus of refugees, mostly heading for Malaysia.
"We know of at least 485 people who've drowned or are lost at sea," said Vivian Tan, spokeswoman for the UN refugee agency, adding the real death toll was probably far higher.
"These numbers are very worrying," Tan said.
"The fact that even women and children are increasingly risking this journey shows the growing sense of desperation among the Rohingya in Myanmar and Bangladesh," she added.
More than 10,000 people have attempted the sea voyage since October 2012 -- a sharp increase on last season's departures, according to the Arakan Project, which lobbies for the rights of the Rohingya, considered by the UN to be one of the most persecuted minority groups on the planet.
"It's already more than the previous boat season, which itself was the most we'd seen," with more than 8,000 people making the journey, according to Arakan Project director Chris Lewa.
She said the estimate does not include boats that left the Rakhine state capital Sittwe, where tens of thousands of Rohingya are living in camps, as her NGO is not able to monitor those departures.
October is the end of the monsoon season and traditionally marks the start of an annual wave of migration by people trying to reach Malaysia from the Bay of Bengal -- often on rickety wooden boats.
The latest exodus shows "the urgent need for countries in the region to respond to this humanitarian crisis by keeping their doors open to people in need of international protection from Myanmar", the UN's Tan said.
Myanmar views the roughly 800,000 Rohingya in Rakhine as illegal Bangladeshi immigrants and denies them citizenship. Thousands more live in squalid refugee camps across the border in Bangladesh.
Malaysia has become the sole hope for many Rohingya refugees, after Bangladesh closed its shared border to them and Thailand as well as Singapore refused to provide asylum to members of the Muslim-minority group.
Kuala Lumpur expressed concern at the influx of refugees, saying Malaysia's patience was being tested.
"There is the humanitarian aspect," Malaysian Foreign Minister Anifah Aman told AFP on Friday, citing the recent rescue of 40 shipwrecked Rohingya who were turned away by Singapore.
"But we cannot allow Malaysia to become a destination of choice," he added, noting that the country was already sheltering some 80,000 Rohingya.
About 500 Myanmar boat people swam to shore in Malaysia last weekend after a 15-day sea journey, according to police there. Another died after being hit by a boat propeller when he jumped into the sea.