He noted that Ri, 69, won his major promotions at a September 2010 party conference but received none 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, Mr Pinkston said.
Ri's departure comes as Kim Jong-un is making his mark as North Korea's new leader in other ways. Last weekend, state TV showed him at a watching a concert and visiting a kindergarten in the company of a mysterious woman who carried herself much like a first lady. Her identity has not been revealed, but making her presence public was a notable change from Kim Jong Il's era, when his companions were kept out of official media.
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.
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 July 6 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 tasked with 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 named vice marshal in April. Ri had been anointed as Kim's patron during the young man's rise to power, Koh said. "But after Kim formally took power, Choe has emerged as No 2."
"Perhaps (Ri) was always meant to be a transitional regent figure, and his function is played," Mr Delury said.
Little is known about Hyon, the career officer newly named a vice marshal. According to the Unification Ministry-affiliated Information Center on North Korea in Seoul, Hyon was named a member of the Central Committee of the Workers' Party in September 2010. In one sign of his rise, he served on the funeral committee for Kim Jong Il in December.
Where Ri's departure and the apparent rise of younger generals leaves North Korea's army remained unanswered.
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 U.S.-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 Korean Peninsula has remained locked in a state of war and divided since a truce in 1953 ended three years of fighting.
The United States 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 said North Korea should "take the path available to it" and rejoin the international community by refraining from threats and complying with its international obligations including denuclearisation. He urged the North to feed and educate its people rather than pour "scarce resources into nuclear, missile and other military programmes."
Source: agencies
