updated: Feb 24 2021, 21:14 ist North Korea used South Korean prisoners of war and their descendants through several generations as slave labour in a vast network of coal mines, a rights group said Thursday. Tens of thousands of South Korean prisoners of war were never returned by Pyongyang after the 1950-53 Korean War. Instead, they were assigned to toil at coal mines in slave-like conditions, with their children and grandchildren inheriting the brutal fate, according to the Seoul-based Citizens' Alliance for North Korean Human Rights (NKHR). North Korea's so-called "songbun" system classifies citizens according to their socio-political background, from the loyal "core" to the "neutral" and the "hostile".