BANGKOK, May 12 -- Labour brokers duped Cambodian men with promises of work in Thailand, trapping them on fishing boats and forcing them to work more than 20 hours a day, lawyers said as a court heard evidence against three defendants accused of human trafficking.

The case in Thailand's southern Ranong province against the owner of a fishing dock and two boat captains comes as the Southeast Asian country steps up efforts to crack down on slavery in its multibillion-dollar seafood industry.

A defence attorney representing the dock owner said allegations against his client were false.

The legal team representing the alleged victims said the men - 15 in all - were hired at a recruitment agency in the Cambodian capital Phnom Penh to clean fish on shore in Thailand, but then found themselves stuck on a boat docked in Mahachai, a seafood-processing city southwest of Bangkok.

"The broker said, 'wait at this boat, don't go anywhere, more workers are coming'. They waited on the boat for one month," said Papop Siamhan, a lawyer and project coordinator for the Human Rights and Development Foundation (HRDF) rights group.

Nine of the men eventually fled and turned themselves in to police so they could be deported home, only to be returned to the broker, Papop told the Thomson Reuters Foundation.

Their broker told them if they wanted to return home to pay 30,000 baht ($850) in order to cover the costs of their documents and travel to Thailand, Papop said.

They stayed on the boat, and then were split up onto two boats which sailed for about a month away from Thailand, and then they began fishing, working seven days a week, some days from 11 p.m. until 8 p.m. the following day.

They were out at sea for more than a year, Papop said. Their plight was discovered in January when Thai authorities called ships to port, and among the boats that came in were the two on which the Cambodian men said they had been enslaved.

After they were released, they each received 25,000 baht ($700) from their employer, though they were each due about 150,000 baht ($4,250) in wages, Papop said.

Defence attorney Supachai Singkalawanich called the victims' tale "impossible".

"They've concocted this story. All 15 people say they are victims and were gone for 13 months, working every day, with no days off, working 22 hours a day, sleeping just two hours a day," Supachai said by telephone from Ranong on Wednesday.

"Working everyday - 22 hours a day without rest - this simply is not possible," he said after the second day of court.

The case was filed on Tuesday at the court, which accepted the case and immediately heard victim testimony, said Preeda Tongchumnum, another lawyer on the case, who also works with the Solidarity Center, a U.S.-based worker rights organisation.

Thailand has come under fire after a series of reports in recent years uncovered widespread human trafficking and slavery in its seafood industry.

The government has recently amended its laws in an effort to combat human trafficking and slavery, as well as illegal, unreported and unregulated fishing.

Last March, Thailand's parliament voted overwhelmingly to introduce harsher punishments for human traffickers, including life imprisonment and the death penalty in cases where their victims had died.