“We only took a Friday afternoon off twice a month but had to spend the time studying books or watching videos about the greatness of our leader back home,” Mr. Rim said at a recent news conference in Seoul, the South Korean capital. “We were never paid our wages, and when we asked our superiors about them, they said we should think of starving people back home and thank the leader for giving us this opportunity of eating three meals a day.”

Tens of thousands of North Koreans work long hours for little or no pay, toiling in Chinese factories or Russian logging camps, digging military tunnels in Myanmar, building monuments for African dictators, sweating at construction sites in the Middle East or aboard fishing boats off Fiji, according to former workers and human rights researchers.

For decades, North Korea has been accused of sending workers abroad and of confiscating most of their wages. But in the years since Kim Jong-un took over as leader, human rights researchers say, the program has expanded rapidly as international sanctions have squeezed the country’s other sources of foreign currency, like illicit trading in missile parts.

A 2012 study by the North Korea Strategy Center, a group in Seoul that works with North Korean defectors, and the private Korea Policy Research Center estimated that 60,000 to 65,000 North Koreans were working in more than 40 countries, providing the state with $150 million to $230 million a year. That number has since grown to 100,000, human rights researchers said.

“North Korea is exploiting their labor and salaries to fatten the private coffers of Kim Jong-un,” said Ahn Myeong-chul, head of NK Watch, a human rights group in Seoul. “We suspect that Kim is using some of the money to buy luxury goods for his elite followers and finance the recent building boom in Pyongyang that he has launched to show off his leadership.”