LHOKSUKON, Indonesia/LANGKAWI, Malaysia (Reuters) - Several thousand migrants, many of them hungry and sick, are adrift in boats in Southeast Asian seas and governments of the region must rescue them quickly to avert a "massive humanitarian crisis", the United Nations said on Tuesday.
It appealed to authorities in Thailand, Malaysia and Indonesia not to push back boats that are packed with refugees who have been abandoned by smugglers following a Thai government crackdown on traffickers.
There has been a surge in migrants from impoverished Bangladesh and Myanmar to Malaysia and Indonesia following the clampdown in Thailand, usually the first destination in the region's people-smuggling network.
Many of the arrivals are Rohingya, a stateless Muslim minority from Myanmar described by the United Nations as one of the most persecuted minorities in the world.
Jeffrey Savage, who works for the U.N. High Commissioner for Refugees (UNHCR) in Jakarta, said that perhaps thousands of people were stranded in the waters between Malaysia and Indonesia.
"What we're hearing from these people is that they've been stuck out at sea for weeks and months and then the smugglers just deserted them, left them with very little food and water, no fuel for the engines, so they're drifting," he told Reuters on the northern coast of Sumatra in Indonesia, where several hundred arrived in boats last weekend.
He said the UNHCR was calling for an international search and rescue operation between the Andaman Sea and the Straits of Malacca to respond to what he termed "a massive humanitarian crisis waiting to happen".
Thailand announced on Tuesday that it was organizing a meeting of 15 countries, to be held in Bangkok on May 29, to address "the unprecedented increase of irregular migration across the Bay of Bengal in recent years".
"Countries of origin, transit and destination must work together to address the problem comprehensively by addressing the root causes as well as all the contributing factors ..." it said in a statement.
An estimated 25,000 Bangladeshis and Rohingya boarded rickety smugglers' boats in the first three months of this year, twice as many in the same period of 2014, the UNHCR has said.
Most landed in Thailand, where they were held by the smugglers in squalid jungle camps until relatives paid a ransom.
Thailand ordered a clean-up of suspected traffickers' camps last week after 33 bodies, believed to be of migrants from Myanmar and Bangladesh, were found in shallow graves near the Malaysian border.
That has led to many migrants being left out at sea.
"Up to 8,000 people are at sea, of which more than 1,000 have landed," said Joe Lowry, regional spokesman for the International Organization for Migration.
"We believe there are no more departures from the Bay of Bengal because of a crackdown by Thai authorities but those who are still at sea have been there for weeks or even months."
The UNHCR estimates that around 300 people died at sea in the first quarter of this year as a result of starvation, dehydration and abuse by boat crews.
Malaysia, one of Southeast Asia's wealthier economies, has long been a magnet for illegal immigrants, but the sudden surge has presented it with a pressing problem.
Boats carrying altogether more than 1,000 people landed on the Malaysian holiday island of Langkawi, close to the Thai border, at the weekend. A senior maritime official there said any more boats trying to land would be turned back.
"We don't allow them in," said First Admiral Tan Kok Kwee, northern region head of the Malaysian Maritime Enforcement Agency. "It's a policy matter."
In Geneva, UNHCR spokesman Adrian Edwards told a news briefing: "Push-backs are not life-saving measures".
On Sunday, nearly 600 migrants thought to be Rohingya and Bangladeshis were rescued from overcrowded wooden boats stranded off Indonesia's Aceh province, believing they had landed in Malaysia, authorities said.
Muhammad Husein, who was on one of those boats, said he and about 80 others who had sailed from Myanmar came near Thai shores after two months at sea.
"As soon as we got close to Thailand we got arrested," he said. "I don't know if the people who arrested us were navy officials or sea patrols or traffickers. They were not wearing uniforms but they had guns."
Husein said they were kept confined in the boats for the next 20 days and given only small amounts of food and water.
"We could see the land, we could see hills, but they would not let us off the boat," he said. "Many of us cried constantly but if we asked for water, we were beaten. They even beat the women."
Last week, he said, his group and many other migrants were transferred to a bigger ship and left to drift on the high seas.
Additional reporting by Reza Munawir in LHOKSUKON, Indonesia, Amy Sawitta Lefevre in BANGKOK, and Stephanie Nebehay and Tom Miles in GENEVA; Writing by John Chalmers; Editing by Nick Macfie