According to Yonhap news agency citing a South Korean parliamentary official, Kim Yong Chol has been recently replaced by Jang Kum Chol, a barely known politician in his late 50s who has a career of working for civilian exchanges between the two Koreas.
Earlier in the day, Kim Yong Chol did not accompany Kim Jong-un in his visit to Russia's Far East city of Vladivostok for a summit with President Vladimir Putin, the agency stressed. It was the first time Kim was absent in the leader's entourage during an overseas trip.
Nuclear negotiations between the United States and North Korea came to deadlock after the second summit in Hanoi when Pyongyang said it wanted Washington to lift all sanctions on the country before it started denuclearising and then faced US refusal.
On Wednesday, the Kremlin said that Russian President Vladimir Putin and North Korean leader Kim Jong-un would meet at about 1 p.m. Vladivostok time (03:00 GMT) on Thursday.