The North is referring to a group of 13 North Koreans who defected to South Korea en masse in April 2016. The 12 female workers and one male manager had worked at a restaurant in China before escaping the country.

Pyongyang has claimed that Seoul's spy agency lured them, calling for an immediate repatriation, but South Korea said that they defected on their own free will.

North Korea said that families of the adbuctees previously sent similar letters to former UN chief Ban Ki-moon and top UN officials on human rights, but they did not get any reply.

"We cannot but express our disappointment at the fact that no action has been taken by the UN until now as we have entered 2017 without even a reply to our letters," the KCNA said.

North Korea claimed that Seoul's abduction constitutes a "crime against humanity" as the move resulted in the separation of families against their will.

"How to deal with this case will be a touchstone testing the true stand of the UN for the promotion and protection of human rights," the letter stressed.

Seoul's unification ministry said in August that the restaurant workers have begun to settle down in the South after undergoing a months-long assimilation program. They were, in addition, questioned about the reason for their defection. 

North Korean workers toiling overseas are pressed to send hard currency to the regime which faces economic hardship under a set of UN sanctions. (Yonhap)