Thai soldiers are detaining illegal migrants from Bangladesh and Burma and forcing them back out to sea in boats without engines, survivors say.
Survivors say their hands were tied and they were towed out to sea with little or no food or water.
About 500 migrants are now recovering from acute dehydration in India's Andaman islands and the Indonesian province of Aceh.
Thai officials were not immediately available for comment.
But sources in the police and army confirmed to the BBC's Jonathan Head in Bangkok that asylum seekers are being pushed out to sea. They did not provide further details about the practice.
Thousands of poor Burmese and Bangladeshis try to reach south-east Asian nations in search of work.
Survivors rescued by Indian coast guards say hundreds of other asylum-seekers are still missing after leaving Bangladesh and Burma since the end of November.
They told the BBC that they paid agents to take them to Thailand by boat so that they could have a better life.
They said that the Thai authorities detained many of them in Koh Sai Daeng island.
"Thai soldiers tied up our hands and then put us in boats without engines. These were towed into the high sea by motorised boats and left to drift," said Zaw Win, a survivor rescued by Indian coast guards off the coast of Little Andamans after drifting for 12 days.
"We were without food and water. The Thai soldiers clearly wanted us to die on the boats," Win told the BBC by telephone from a camp where survivors are being cared for.
Other survivors said that about 400 migrants were put on a huge boat by soldiers. It was equipped with only two bags of rice and two drums of drinking water.
"The food and water ran out in two days. After that we were starving for nearly 15 days before we saw a lighthouse and jumped into the sea and tried swimming ashore," Mohammed Said told the BBC.
This group of migrants was also rescued by the Indian coast guards and put into relief camps.
"They have all suffered huge dehydration. We are taking care of them the best we can," said Ratan Kar, deputy director of health services in the Andamans.
'Dehydration and starvation'
Nearly all of those rescued have equally harrowing stories.
One Rohingya villager from Burma said that his son and seven friends had left together on the same boat.
He said that after they were arrested by the Thai authorities, they were forced onto the same large boat without an engine:
"Four of them, including my son, survived but four died," he said.
"My son told me that many died because of dehydration and starvation but many also jumped into the sea.
"When the boat finally drifted close to an Andaman island, there were only just over 100 still onboard."
The refugees say that hardly any of them escaped the Thai military guarding the country's coastal islands.
Human rights activists have condemned Thailand's "inhuman and brutal response" to this new wave of illegal migration.