SEOUL - North Korea's army chief has been removed from all his posts due to illness, state media announced Monday, which analysts said showed new leader Kim Jong-Un tightening his control over the powerful military.

Ri Yong-Ho is regarded as one of the key figures who supported the young, untested leader in the transition following the death in December of his father Kim Jong-Il who ruled the reclusive state for 17 years.

The regime's quick announcement on Ri, 69, was "very unusual", prompting Seoul to keep an eye on the North's next move, said Kim Hyung-Suk, spokesman for the South's unification ministry handling cross-border affairs.

Observers say the outgoing vice marshal may have fallen out of favour with Kim Jong-Un.

Ri became head of the North's 1.2 million-strong armed forces - one of the world's largest - in 2009 and had often been seen accompanying Kim Jong-Un on visits to military bases this year.

The North's official KCNA news agency said top officials of the Workers' Party of Korea took the decision Sunday to relieve the vice marshal of his posts due to illness.

He was removed from the political bureau presidium, the party's highest body with a handful of members, and his post as a vice-chairman of the central military commission, it said.

The general was one of seven top party and military cadres who accompanied Kim Jong-Un when he walked alongside the hearse carrying the body of his father Kim Jong-Il during his funeral.

The seven - including Kim Jong-Un's uncle Jang Song-Thaek - were considered central figures in bolstering the regime of the new leader, who is in his late 20s.

Ri was also seen accompanying Kim Jong-Un when he paid tribute to his late grandfather Kim Il-Sung on the July 8 anniversary of his death in 1994.

Professor Yang Moo-Jin of the University of North Korean Studies in Seoul said he was sceptical about the reason given for the "hawkish" veteran field commander's departure.

"He might have fallen into disfavour with Kim Jong-Un or lost in a power struggle with other military leaders," he said, adding Pyongyang seldom relieved party or military leaders simply for health reasons.

Paik Hak-Soon of the Sejong Institute said Kim Jong-Un might be seeking to strengthen the party's control over the military which had become too powerful under the "Songun" (army-first) policy of his father.

"Jong-Un will make sure that now the party keeps the overgrown military under check - an effort his father started in late 2010 before he died," he said.

"Ri is an old fixture from the father's generation. Jong-Un will likely replace him with someone younger and closer to the party ... someone he can control more easily," Paik said.

Cheong Seong-Chang, another Sejong researcher, said Ri must have been ousted after protesting at the party's increasing attempt to control the military.

"The latest dismissal shows that even a figure like Ri, who has enjoyed the deep trust of Kim Jong-Un, can be fired overnight," he said, predicting the party's control over the military would grow.

The North's military has in recent months ratcheted up hostile rhetoric towards South Korea and its President Lee Myung-Bak, partly in a bid to burnish Kim Jong-Un's credentials.

Ri, at a massive anti-Seoul rally in Pyongyang in March, called South Korean leaders "mad dogs" and "psychos" and declared a "sacred war" against Seoul for allegedly insulting the North's leadership.

The impoverished but nuclear-armed North last month also denounced US-South Korean drills near the tense border as a provocation and vowed to bolster its "nuclear deterrent".

It was the latest sign of high tensions after the North's failed rocket launch in April, seen by the United States and its allies as an attempted ballistic missile test.