North Korea has named a new vice marshal in the army, state media reports, a day after it emerged that the country's army chief had been relieved of duties.

The decision to appoint Hyon Yong-chol was taken on Monday by the ruling party's central military commission and the country's national defence commission, the official news agency KCNA said.

Little is known about Hyon, who became a general in September 2010 along with five others including new leader Kim Jong-un and his aunt Kim Kyong-hui.

He is a member of the party's 120-member central committee. But he has not joined the central committee's powerful military commission, which is chaired by Kim.

State media said on Monday that Ri Yong-ho had been relieved of all his posts due to illness.

He was regarded as one of the key figures who has helped support the young Kim in the transition following the death in December of his father Kim Jong-il, the longtime leader of the reclusive state.

Ri, who became head of the army in 2009 with the official title Chief of the General Staff of the Korean People's Army, had often been seen accompanying the young Kim on visits to military bases in recent months.

South Korean officials said they were surprised at the speed of Ri's dismissal, and that it was done so publicly.

"We will keep monitoring closely the situation in the North," said Kim Hyung-suk, the South Korean Unification Ministry spokesman.

Ri was one of seven top party and military cadres who accompanied Kim when he walked alongside the hearse carrying the body of Kim Jong-il during his funeral.

The seven - including Kim's powerful uncle Jang Song-thaek - were considered central figures in bolstering the new regime led by Kim, who is believed to be in his late 20s.