Brazil has made considerable progress in freeing slave laborers but has not done enough to punish the crime or tackle its causes, experts told a congressional committee on Thursday.

Brazil's booming farm sector has repeatedly been accused by foreign competitors of ignoring international environmental and labor standards.

Most slave labor occurs along the agricultural frontier encroaching on the Amazon rain forest, where labor is needed to clear huge tracts of land to plant crops or graze cattle.

Poor migrants looking for jobs are usually taken by bus or by trucks to farms, where they fall into debt peonage. Often, their documents are taken and armed guards prevent them from leaving.

Brazil has stepped up controls since former union leader Luiz Inacio Lula da Silva came to power in 2003, freeing 15,875 forced laborers, nearly five times the annual average between 1995 and 2002.

"Without a doubt Brazil has made big advances," Patricia Audi of the International Labor Organization, or ILO, told a congressional human rights committee on Thursday.

She cited a black list that banned farmers who used slave labor from receiving government credit and an accord companies signed in 2005 to reject products made with slave labor.

Audi also praised a mobile inspection unit that carried out surprise raids on farms.

But critics said such raids may be uncovering only a fraction of actual slave labor.

The CPT, a rural watchdog linked to the Catholic church, said the number of reports it receives denouncing inhumane working conditions has remained steady at 200 to 300 a year, involving between 6,000 and 8,000 workers.

"The government is investigating only half of all the cases," Father Xavier Marie Plassat of the CPT told Reuters on the sidelines of the committee hearing. "We still have a long way to go."

Education, jobs and land-reform were needed to eradicate the extreme poverty that gave rise to slave labor, he said.

Brazil needed harsher fines, longer prison sentences, and judicial enforcement of penalties, experts said.

"So far, nobody has been punished for holding workers in slave-like conditions," Luis Antonio Camargo de Melo, the country's chief labor prosecutor, told the committee.

"It's an embarrassment to all of us," he added.