Secretive North Korea on Thursday published a photograph of the youngest son and heir apparent to the communist state's ailing leader Kim Jong-Il, the first official image of him ever released.

The photo of Kim Jong-Un was taken after the ruling party's highest-level meeting for 30 years, which bestowed powerful posts on him in a clear sign he is being groomed for the next dynastic succession in the communist state.

Official media carried a photo of leading ruling party officials. Party newspaper Rodong Sinmun listed the son Jong-Un as one of those in the group.

It did not say where he is seated. South Korean experts and a government official identified a chubby young man sitting next but one to the leader as Jong-Un.

"We believe he is Kim Jong-Un," said unification ministry spokeswoman Lee Jong-Joo of the man dressed in a dark Mao-type suit.

The young man who seems set to take over the impoverished but nuclear-armed nation is a mystery to the outside world. No adult photo has previously been seen outside the North.

"The publication of his picture is tantamount to a declaration that Jong-Un is the heir apparent," Professor Yang Moo-Jin of Seoul's University of North Korean Studies told AFP.

"This is also a signal that the junior Kim is launching official activities."

Yang said Jong-Un takes after his grandfather Kim Il-Sung but is short and stout like his father.

The United States and other nations are scrambling for more information about the Swiss-educated Jong-Un, believed to be aged about 27.

However a North Korean ruling party delegation left on Thursday on a visit to the country's only major ally China, state media said.

According to South Korean media the delegation was likely to brief Chinese officials on the party meeting.

There was no indication that Kim Jong-Un was among those travelling.

US Central Intelligence Agency Leon Panetta will arrive in South Korea Saturday to exchange information about the succession, Yonhap news agency reported.

The son's name was not mentioned by official media until this week, when leader Kim appointed him a four-star general just before Tuesday's meeting of the Workers' Party of Korea.

Analysts say the North will likely seek to ease overseas tensions as it prepares for an eventual power transfer from the ailing 68-year-old leader to his son.

But the first inter-Korean military talks for two years ended without progress Thursday as Seoul demanded an apology from Pyongyang for the deadly sinking of a South Korean warship, the South's defence ministry said.

South Korean officers "strongly urged North Korea to admit to, apologise for and punish those responsible for the attack on the Cheonan warship", the ministry said in a statement.

It also demanded the North "immediately stop its military threats and aggressive behaviour at sea borders".

The North refused to accept the findings of a multinational investigation which blamed the March sinking and the death of 46 sailors on a North Korean torpedo.

No date was set for the next round of talks.

After months of high tension over the ship, the North has lately made apparent conciliatory gestures to South Korea and the United States.

But it still vehemently denies involvement in the naval tragedy and describes joint US-South Korean naval exercises being staged as a show of strength as a rehearsal for attack.

South Korean officials "remain unchanged in their ulterior intention to harm the (North), backed by their American master," cabinet newspaper Minju Joson said Thursday, accusing Seoul of trying to spark a nuclear war with the latest joint drill this week.

Defence Minister Kim Tae-Young told a forum Seoul has detected signs of possible provocations by North Korea, especially in 11 border areas where the South has set up propaganda loudspeakers as part of reprisals for the warship sinking.

They have not yet been switched on.

Kim said Pyongyang "seems to be mainly focusing on forming a platform for power succession and easing its food shortage and economic troubles."

The North has expressed conditional willingness to return to stalled six-nation nuclear disarmament talks. But it wants a US commitment to hold separate talks on signing a permanent peace treaty.

"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," Pyongyang's Vice Foreign Minister Pak Kil-Yon told the UN General Assembly Wednesday.

burs-njc