Ri Yong-ho, North Korea's military chief and key adviser to the new leader Kim Jong-un, has been removed from all posts, according to state media reports.
The North's state-run KCNA reported that Ri was relieved of all duties "because of illness," an explanation which was viewed with skepticism by many outside Pyongyang. The decision was taken at a meeting of the ruling Workers' Party Central Committee.
"A meeting of the political bureau of the central committee of the workers' party decided to relieve Ri Yong-ho of all his posts for his illness," said the terse statement appearing in KCNA.
No announcement has been made about the illness of the outgoing military chief or about his successor.
Ri was vice-chairman of the powerful Central Military Commission and the chief of the military's general staff. He also held top positions in the Workers' Party.
He was thought to be one of the key figures in the inner circles of the present leader Kim Jong-un and seen as supportive of the new leader, who was considered young and inexperienced.
The latest move by the young leader has surprised political observers as they try to analyse the meaning of his actions.
Ri became army chief in 2009 under the current leader's father, Kim Jong-il, who passed away in December 2011. Ri had been photographed with Jong-il on several state occasions. He was also present during the funeral ceremony of the late leader.
Ri was one among seven high ranking officials who accompanied Jong-un as his father's body was taken to the funeral ceremony.
Ri was also seen with Jong-un at military bases and several recent state functions.
Recent reports, denied by Pyongyang, suggested that North Korea was in the final stages of a third nuclear test.