Underscoring the concern, a migrant organisation said a vessel was adrift somewhere near the Thai or Malaysian coast with around 350 people -- including women and children -- but no food or water, while Thailand called a regional summit on the issue.
Indonesia's navy, however, said earlier it had turned away one boat carrying hundreds of migrants from Myanmar and Bangladesh, whose fate remains uncertain.
Dhaka, meanwhile, detained a trawler after it was cast adrift with 116 Malaysia-bound illegal migrants on board.
Many of them were thin, weak or in poor health after weeks at sea.
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The Arakan Project, a group advocating for the rights of Muslim Rohingya says as many as 8,000 other people may be adrift.
The group said it had spoken by phone with passengers aboard the vessel carrying 350 people, who said they were abandoned by their Thai human-traffickers.
"They told us they have had no food and water for the last three days. They have called for urgent rescue," said Chris Lewa, the group's founder.
Like thousands from the Rohingya community, he said he was "running for my life" from sectarian violence in Buddhist-majority Myanmar.
He and about 800 other people endured 43 days on an overcrowded vessel bound for Thailand as meagre food and water supplies dwindled to nothing.
The ship diverted to Malaysia's Langkawi island where the hungry passengers leapt into the sea in a desperate swim to safety.
"There was no more food or water so we just jumped out of the boat," he said.