A record 5,803 foreign trainees went missing in 2015 while working in Japan, the Justice Ministry's Immigration Bureau said Monday, sparking concern that many may have remained in Japan as a source of black market labor.

Observers say this is another sign that the technical intern program — long criticized by rights groups at home and abroad as akin to slavery — is seriously flawed.

According to the ministry, of the 5,803 who went missing, people from China made up more than half, at 3,116. They were followed by 1,705 Vietnamese, 336 people from Myanmar, 250 Indonesians and 102 Nepalese.