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.

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.

Privately, some Thai military and police sources have admitted to the BBC that this has been happening - they say the escalating numbers of Rohingyas reaching Thailand from Burma or Bangladesh are seen as a security risk, because of fears they may include Islamic militants.

The reason they disable their engines, they say, is to prevent them trying to come back to Thailand.

Refugee welfare groups have condemned the practice as inhumane.

The Thai government says it has ordered an investigation of the incident, but stressed that is committed to humanitarian principles in handling illegal immigration.

In one case last month, the reports say, 410 Rohingya migrants were taken out to sea on a Thai Navy vessel and forced onto an open barge with just four barrels of water and two sacks of rice.

Four people were thrown overboard with their hands and feet tied, in order to encourage the others to board the barge, according to the reports.

Navy chief Admiral Kamthorn Phumhiran on Friday dismissed a BBC report claiming Thai military had ill-treated the Rohingyas from Burma and Bangladesh who sought work or asylum by pushing them out to sea and setting them adrift.

"The Royal Thai Navy did not badly treated the Rohingyas. There was no setting them adrift as alleged," he said.

Under the military convention, the navy is obligated to rescue enemies from a sunken ship, he said, arguing there is no reason to mistreat the migrants landing on the Thai shore.

In the incident in question, the navy was notified by marine park rangers about the Rohingyas at Kon Sai Daeng, Ranong, he said.

A group of 20 navymen were dispatched to investigate and they found more than 100 Rohingyas, prompting the order for the migrants to lie down for safety reason, he said.

He added that the picture depicting navymen standing over the rows of migrants lying down should not be construed as mistreatment but a standard procedure to detain a large number of people.

The Rohingyas were brought to a shelter with food and water, he said. Afterwards, the navy alerted the immigration police and handed over the custody.