Thailand says it wants to clean up its act as a hotbed of human trafficking, but those efforts took a hit this week, after a report alleged that government immigration officials are involved in the sale of Rohingya refugees near the tourist mecca of Phuket.

Journalists Chutima Sidasathian and Alan Morison, writing for the Phuket Wan news site and the South China Morning Post, reported this week that a busload of about 50 Rohingya detainees were offloaded from a Thai government immigration bus with bars on the windows in the southern port city of Ranong and then transported to ships offshore. The Rohingya are an oppressed Muslim ethnic group who have been violently persecuted in Myanmar, sending many refugees fleeing by land and sea into Thailand, India and Bangladesh.

“The transfer could not have been a legitimate deportation of Rohingya back to Myanmar since that country refuses to acknowledge members of the Muslim ethnic minority as citizens and won’t accept their repatriation,” Chutima and Morison noted.

Rohingya in Thailand are sold to middlemen for 10,000 baht ($321) each, then offered for sale back to their families for as much as 65,000 baht ($2,087), according to Chutima and Morison, citing interviews with smugglers. If the families cannot pay, the refugees are forced to work in the fishing industry—which, as Quartz has reported, relies heavily on slave labor. Migrants are forced into debt bondage to harvest and process the shrimp and other seafood that is exported to markets like the United States—a $7.3 billion business. A report last week estimated that there are up to 500,000 enslaved people in Thailand.

“The issue is, why do we have to keep the Rohingya in Thailand? Holding them costs nearly three million baht every month,” Colonel Nattasit Maksuwan, the deputy chief of the Internal Security Operations Command in Satun province, said to Chutima and Morrison. Thai immigration officials declined their request for comment.

The new report may put new pressure on the Thai government as it tries to improve its position on the US State Department’s Trafficking in Person rankings. Thailand has been on the Tier Two watchlist for four years and faces an automatic downgrade to Tier Three—defined as “Countries whose governments do not fully comply with the minimum standards and are not making significant efforts to do so”—unless it shows significant improvements in tackling human trafficking. Under US law, Tier Three status mandates sanctions that cut certain types of US foreign assistance.

Thai prime minister Yingluck Shinawatra met with officials last week about improving the country’s standing on the US trafficking list, and “emphasized that police, public prosecutors and the courts should now begin to work more closely together in the fight,” according to a spokeswoman.