The youngest son of ailing leader Kim Jong-Il has taken over powerful posts in North Korea's ruling party, state media said Wednesday, confirming his status as heir apparent in the nuclear-armed nation.

Kim Jong-Un was named one of two vice-chairmen of the central military commission of the Workers' Party of Korea and a member of its central committee, a day after his father appointed him a four-star general.

"Now the crown prince has all he needs to become the next leader," said Choi Jin-Wook, senior analyst with South's Korea Institute for National Unification.

Jong-Un's high position on the military commission chaired by his father gives him considerable authority over one of the world's largest armed forces, numbering almost 1.2 million.

The speed of his ascent may indicate the 68-year-old father's health is worse than believed, some analysts said.

Kim Jong-Il, who took over from his own father and national founder Kim Il-Sung, suffered a stroke in 2008 and is also thought to have kidney problems.

The Swiss-educated son, believed aged about 27, had never been named in state media before this week. No adult photograph has been seen outside the secretive communist nation.

But son and father appeared at a photo session with other newly elected party officials at the plaza of the Kumsusan Memorial Palace in the capital Pyongyang, the North's official Korean Central News Agency reported.

The United States is "watching and looking for details (on the son) just about like everyone else," said Assistant Secretary of State Kurt Campbell, adding he would visit Japan and South Korea next week to discuss the changes.

The appointments were announced after a one-day conference on Tuesday of the communist party, its highest-level gathering for 30 years.

The meeting also appointed the leader's sister Kim Kyong-Hui as a member of the party political bureau.

Her powerful husband Jang Song-Thaek was made an alternate member and also appointed to the military commission and the central committee.

The couple are seen as likely to act as guardians to the untested son as he gains experience.

The Kim dynasty has ruled with an iron fist since the country was founded in 1948 but the current regime faces daunting problems.

It is struggling to revive a crumbling command economy amid persistent severe food shortages and grappling with international sanctions over its nuclear and missile programmes.

Analysts said the North will likely seek to reduce tensions with the United States and South Korea and focus on improving living standards to try to win public acceptance of another dynastic power transfer.

"In order to stabilise the succession plan, the North must ease economic difficulties and for this purpose, improving ties with the outside world is essential," Koh Yu-Hwan, of Seoul's Dongguk University, told AFP.

South and North Korea will Thursday hold their first military talks for two years, Seoul's defence ministry said.

But in a new blow to international efforts to tempt it back to nuclear disarmament talks, North Korea Wednesday vowed to strengthen its atomic arsenal.

Speaking at the UN General Assembly, Vice Foreign Minister Pak Kil Yon said: "As long as the US nuclear aircraft carriers sail around the seas of our country, our nuclear deterrent can never be abandoned, but should be strengthened further. This is the lesson we have drawn."

The timing of a power transfer seems linked to the senior Kim's health.

Analyst Choi said Jong-Un's promotions meant "Kim Jong-Il's health is really in a dire condition and he is trying hard to clear up uncertainties surrounding the future of the regime's leadership."

However, Yang Moo-Jin, of Seoul's University of North Korean Studies, believes Kim is still relatively healthy but hurrying to install his son because of his own advancing age.

The conference laid the foundation for another hereditary succession but also showed Kim is still firmly in control, Yang said. The party re-elected Kim senior as its general secretary.

Jong-Un's military appointments mean the North will continue its army-first policy, Yang said, adding the son is now in a position to control the armed forces.

But Jong-Un did not become a party secretary in charge of personnel changes or join its prestigious political bureau presidium.

"The leader has apparently decided that the son is still too young and inexperienced and needs more time to prepare himself to be in charge of personnel appointments," Yang said.

burs/mtp