Thousands of Myanmar prisoners die each year from starvation and sickness or are beaten to death in "slave labor camps," a group of released prisoners said Monday.

Myanmar's military government freed 101 Thai prisoners on Monday in what it called a goodwill gesture to boost relations with its eastern neighbor.Most of the Thais are fishermen, locked up for more than three years in Yangon's In Sien prison and in Mandalay after their vessels entered Myanmar's territorial waters.

The Thai prisoners, speaking on their return to Bangkok, said they were generally treated well but spoke of appalling conditions faced by their Myanmar cellmates, most of whom were convicted of petty crimes such as theft.

Local prisoners faced regular beatings, infection from HIV-contaminated medical supplies or were sent to labor camps from which many never returned, they said.

"Myanmar tries to lessen overcrowding in its jails by sending prisoners to slave camps from which only 50 percent return," one Thai trader, who spent time in both In Sien and Mandalay jails, told Reuters.

The released prisoners said Mandalay jail, which houses more than 9,000 inmates, sent prisoners to work in rice fields or on construction sites where they were forced to break rocks.

"My cellmate who returned from six months in a slave camp told me that only 70 of 300 prisoners returned to jail," said a prisoner who was sentenced to 18 years for attempting to smuggle rubies out of Myanmar.

"More than 50 percent died from poor nutrition or sickness while some escaped," he added.

He said officials and fellow inmates had told him during his 10-month stay in Mandalay that at least 50 prisoners died in the jail each month from sickness or beatings.

One released trader said more than 530,000 prisoners were kept in Myanmar's relatively few jails. "From Yangon to Mandalay, Myanmar has only 10 jails," he said.

The Thais said the worst ordeals were experienced in In Sien jail in Yangon, both by local and foreign inmates.

"Five hundred prisoners were sent out to slave camps in May, but less than 100 of them returned. They looked like skeletons, not like human beings," said another ex-prisoner.