North Korea's Central Guidance Bureau for Special Zone Development has reportedly sent a letter to the head of an association of South Korean firms operating in the North's Gaeseong Industrial Complex.

In the letter sent to Kim Hak-kwon Saturday, the North claimed that the crimes of a detained South Korean worker are "grave," and the conflicts on the operation of the complex have been caused by Seoul, which has failed to implement the June 15th Joint Declaration.

The association said Thursday that Pyongyang seemed to convey its positions in the letter after the association held an emergency meeting on June 25th and called on the North to guarantee free passage as well as the safety of South Korean workers at the complex.