North Korea is reshuffling its most powerful institution, the military, dismissing its army chief, a key mentor to young ruler Kim Jong-un, and promoting a little-known general to an important position in the million-man force.

Illness was the reason cited for Ri Yong Ho's departure, but to some outside analysts it resembled a purge by Kim as he tries to shape the government he inherited seven months ago. The announcement on Tuesday of Hyon Yong Chol's promotion could further that goal; his is the fourth vice-marshal appointment North Korea has made public since the death of Kim's father, Kim Jong-Il.

The changes have significant but as yet unclear implications for North Korea's relationship with its neighbours and the US, which stations more than 28,000 troops in ally South Korea. The authoritarian nation maintains one of the world's largest armies, has built up its nuclear weapons and missile programmes despite broad condemnation and sanctions, and regularly flings warlike rhetoric at Seoul and Washington.

News of Hyon's promotion in the Korean People's Army followed the announcement on Monday that Ri, a vice-marshal who had been chief of the general staff of the army since 2009, was dismissed from his high-ranking posts in the military and the Workers' party because of illness, according to state media. No details were provided about who might succeed Ri as army chief.

Ri had been at Kim Jong-un's side throughout his transition to leadership and after his father's death, and appeared healthy in a public appearance just days ago. Analysts were sceptical about the official explanation for his abrupt departure.

"There's a very high probability that it wasn't health issues, but that he was purged," said Daniel Pinkston, a North Korea analyst at the International Crisis Group.

He noted that Ri, 69, won his major promotions at a September 2010 party conference but received none at another major conference in April, stirring speculation about his future. Even if Ri never directly defied the new leader, his departure would send a strong warning to anyone seeking to challenge Kim Jong-un, Pinkston said.

The dismissal of the top army official is a significant move in North Korea. Kim Jong-Il elevated the army's role when he became leader after the 1994 death of father Kim Il Sung, the nation's founder.

Kim Jong-un has upheld his father's "songun" military-first policy, but in April he also began promoting younger officials to key military and party posts.

North Korea's political and military reshuffles are mysterious, with officials sometimes dropping out of sight without explanation or their departures blamed on illness.

Bruce Klingner, a North Korea analyst at the Heritage Foundation thinktank in Washington, said the shakeup was cause for concern, whether Kim Jong-un was solidifying his power or dealing with a direct challenge to his leadership.

"North Korean leadership instability is worrisome to the United States and its allies since it increases the potential for volatility, additional provocative acts, or implosion of a regime possessing nuclear weapons," Klingner said.

The robust and stocky Ri showed no sign of illness when he spoke in late April at a meeting of top officials marking the 80th anniversary of the army's founding. He was shown in photos on 6 July chatting with Pyongyang residents and two days later joined Kim Jong-un at the Kumsusan mausoleum to pay respects to Kim Il Sung.

"Whether because of a physical malady or political sin, Ri Yong Ho is out, and Pyongyang is letting the world know to not expect to hear about him anymore," said John Delury, an assistant professor at Yonsei University's Graduate School of International Studies in South Korea.

Ri's departure could mean he lost a power struggle with rising star Choe Ryong Hae, the military's top political officer given the task of supervising the army, said Koh Yu-hwan, a professor at Seoul's Dongguk University.

Choe was promoted to several top posts and was one of three new vice-marshals North Korea announced earlier this year.

"Perhaps (Ri) was always meant to be a transitional regent figure, and his function is played," Delury said.

Little is known about Hyon, the career officer newly named a vice-marshal. According to state media, Hyon was named a member of the Central Committee of the Workers' party, a top decision-making body, in September 2010. In another sign of his rise, he served on the funeral committee for Kim Jong-Il in December.

The reshuffle comes amid North Korean threats in recent months to attack South Korea's president and Seoul's conservative media, angry over perceived insults to its leadership and US-South Korean military drills that Pyongyang says are a prelude to an invasion. A North Korean artillery attack in 2010 killed four South Koreans.

The US said on Monday that without fundamental change in policy direction, personnel changes in North Korea's military leadership would mean little.

State department spokesman Patrick Ventrell urged the north to feed and educate its people rather than pour "scarce resources into nuclear, missile and other military programmes".