North Korean leader Kim Jong Un's most trusted policy adviser apparently has been removed from one of his posts, a South Korean politician has said.
The possible personnel reshuffle comes in the wake of the breakdown of the North Korea-US summit in February and as the North Korean leader visits Russia.
The head of the South Korean parliament's intelligence committee, Lee Hye-hoon, cited South Korea's main spy agency as saying that Kim Yong Chol lost his Workers' Party post in charge of relations with South Korea earlier this month.
He was replaced by the little-known Jang Kum Chol as the director of the party's United Front Department, Lee said. Ms Lee said she obtained the information at a private briefing from the National Intelligence Service.
Kim Yong Chol has been North Korea's top nuclear negotiator and counterpart of US secretary of state Mike Pompeo since Kim Jong Un entered nuclear talks with the US early last year.
He travelled to Washington and met President Donald Trump twice before Mr Kim's two summits with Mr Trump.
His rise had baffled many North Korea watchers because he handled South Korea ties, not international or US relations.
Previously, he was a military intelligence chief believed to be behind a number of provocations, including two deadly attacks in 2010 that killed 50 South Koreans and an alleged 2014 cyber-attack on Sony Pictures.
Both Seoul and Washington imposed sanctions on him in recent years. The NIS and the Unification Ministry, a Seoul agency responsible for North Korea ties, said they could not immediately confirm the information on Kim Yong Chol.
The NIS has a mixed record in confirming developments in North Korea. But if confirmed, Kim Yong Chol's replacement would add to speculation that he is being sidelined from nuclear diplomacy to take the responsibility for the failure of the February summit in Hanoi.
Kim Jong Un, who is desperate to revive his country's moribund economy, returned home empty-handed from Hanoi after Mr Trump rejected his calls for easing US-led sanctions in return for dismantling a key nuclear complex, a limited denuclearisation step.
Kim Yong Chol was not among a list of officials accompanying Kim Jong Un on his current visit to Russia, which began earlier on Wednesday.
Many experts in South Korea said North Korean foreign minister Ri Yong Ho and first vice foreign minister Choe Son Hui would take the lead in the nuclear diplomacy.
Cheong Seong-Chang, an analyst at South Korea's Sejong Institute said:
(Pyongyang's) significantly diminished reliance on Kim Yong Chol is a very positive sign for the denuclearisation negotiations between North Korea and the United States.
He called Kim Yong Chol "most responsible" for the second summit's failure due to his hardline stance.
While the NIS believes the personnel change possibly indicates that the department takes a back seat in the nuclear negotiations with Washington going forward, the spy agency also said it was not immediately clear whether Kim Yong Chol would be removed from the talks entirely or immediately, Ms Lee said.
Mr Kim still maintains several other prominent titles, including vice chairman of the Workers Party's Central Committee and a member of the powerful State Affairs Commission.