North Korea has been sending workers overseas for decades and stepped up the practice during the 1990s as the country slipped into economic chaos and famine. It now appears to be expanding again, activists claim, partly as a result of heightened international sanctions that mean the cash-strapped regime is looking for new sources of revenue.
The wages of such workers, paid in foreign currency, provide a stream of income that is seen as vital to keeping Mr Kim's Workers' Party in power.
Accruing foreign currency became of "paramount importance" in 2012 after he took control of a country with "no economic stability", the North Korea Strategy Centre, an activist group, said in a report released that year.
Pyongyang's growing thirst for cash means thousands more North Koreans are being shipped overseas, where, according to rights activists and former workers, their experiences are often tantamount to slavery.
"There is no contract, they say they will give us health insurance and heating access but we never receive anything," one North Korean worker in Russia was quoted as saying in the North Korea Strategy Centre report. "In reality we earn about 300 roubles [pounds 3.10] but they end up taking it all away."
The group named Russia as the biggest recipient of North Korean labour, with at least 20,000 to 25,000 workers. There were an estimated 15,000 workers in the Middle East and the same number in South East Asia. China and Africa had up to 8,000 each.
In China, many North Koreans are sent to work gruelling hours in sweatshops where they are watched over by security guards. In the Middle East, they are often forced to toil in sweltering and potentially deadly temperatures on building sites. When they are not at work they are kept completely isolated from the country around them.
"The North Korean workers are trapped in wired fences so we have never met any other Koreans," a male worker in Kuwait was quoted as saying.
Qatar has faced particular criticism for its alleged exploitation of North Korean workers as it embarks on a construction programme ahead of hosting the 2020 World Cup. There are about 2,800 North Koreans in the tiny Gulf state, some of whom have reportedly been put to work building Lusail, a skyscraper-packed city that is being created to host the World Cup final and is known as Qatar's "Future City". Developers claim their multi-billion-dollar project, which is being built from scratch outside Doha, will be a "modern and ambitious society".
However, activists said that the North Koreans involved in this project and others like it received almost nothing in exchange.
