Victims of human trafficking are being abducted by force and left to survive on boats anchored in international waters, a new report has found.

An investigation by Reuters found that while in the past most people boarded smuggling boats voluntarily, they are now being kidnapped or tricked even at the first stage of the chain.

Reporters interviewed Bangladeshi and Rohingya survivors, some of whom who had been taken to Thailand where human trafficking gangs run "brutal jungle camps" until relatives pay a ransom.

They told stories of being drugged, tied up and blindfolded before being put on small boats which carried them to larger ships at sea.

Forced to survive on scraps and contaminated seawater, hundreds of people live in these "floating prisons" for weeks. Those who die are thrown into the sea.

One victim, 20-year-old Afsar Miae from Bangladesh, was abducted by a gang who had offered what he thought was legitimate work. He ended up on a ship anchored in the Bay of Bengal, which later set sail for Thailand.

Prisoners "were forced to squat for much of their journey and sometimes had their hands and feet bound with rope or cloth," Reputers reports.

"The guards routinely beat them with sticks or whipped them with rubber fan belts. Food was a handful of rice a day, or nothing at all. What little drinking water they received was contaminated with sea water."

Miae and 80 other men were abandoned on a remote island before they had reached their destination, however. Officials suspect that their captors believed the trafficking chain to have been discovered.

"Their conditions were beyond what a human should have to go through," said Jadsada Thitimuta, an official in Phang Nga involved in the rescue mission. "Some were sick and many were like skeletons. They were eating leaves."

Thailand's Ministry of Social Development and Human Security says more than 130 suspected trafficking victims have been found in Phang Nga since October 11. Most are Bangladeshi, though some are Rohingya Muslim from western Myanmar.

The UNHCR has confirmed that "bigger fishing or cargo vessels" are carrying up to 700 passengers across the Bay of Bengal to Thailand, and October is said to mark the beginning of the busiest time for the trafficking industry as the sailing season sets in.

The Royal Thai Navy has admitted it is aware that people are being held captive on ships off its coast.

"The truth is they use fishing boats to transport people and the bottom of the boat becomes like a room to put the people [in], but it seems like a commercial fishing boat," said Royal Thai Navy spokesman Rear Admiral Kan Deeubol.

Officials working on the Banladeshi coast said it's no easier to stop the operation at their end. "At night they enter our waters, take the people and again cross the boundary. It is very difficult to identify those ships at sea," Lieutenant Commander M. Ashiqe Mahmud explained.

Thai offcials say that a crackdown on trafficking has forced traffickers to become "more sophisticated and cautious". However, human rights groups believe that trafficking has become increasingly lucrative, and high competition between smugglers has therefore led them to begin abducting.

Chris Lewa of the Arakan Project, a Rohingya advocacy group said that crime-ring organisers are "desperate" to cash in on the trade.

"There are always five to eight boats waiting in the Bay of Bengal. And the brokers are desperate to fill them," he said.

(Additional reporting by Reuters)