A senior US official held talks in South Korea Thursday about North Korea's apparent preparations for a power transfer, after the heir apparent to Kim Jong-Il made another public appearance.

Assistant Secretary of State Kurt Campbell said Seoul and Washington needed to "remain in lockstep" in responding to developments in the nuclear-armed nation.

He also stressed that the North must mend ties with the South and show commitment to denuclearisation before international disarmament negotiations can resume.

"What's most important... is to ensure the US and South Korea will remain in lockstep and are extraordinarily closely engaged on mutual assessments in terms of developments in North Korea," the top US diplomat for East Asia told reporters.

Campbell, who arrived from Tokyo, said his talks with Deputy Foreign Minister Kim Jae-Shin and chief nuclear envoy Wi Sung-Lac gave him a chance "to discuss developments last week in terms of the party congress in North Korea".

The meeting, the biggest in three decades, gave Kim Jong-Il's youngest son Jung-Un powerful party posts, a day after his ageing father had named him a four-star general.

Pyongyang's official media said Thursday the young protege attended a concert with his father, his second reported public appearance in a week after he cemented his status as leader-in-waiting.

Media said Tuesday that the two watched a military exercise staged to mark a Workers' Party anniversary this coming Sunday. The North is reportedly also planning a huge military parade to commemorate the event.

The likely eventual next leader remains a mystery to the outside world. Until last week the Swiss-educated young man, believed aged about 27, had never had his name or photograph published in official media.

His 68-year-old father suffered a stroke two years ago and is thought since then to have speeded up succession planning.

The South's Unification Minister Hyun In-Taek, in charge of cross-border relations, said the North was entering a "era of major transformation" during succession preparations.

"Such a change bears innate uncertainties politically, economically and socially," he told a forum Thursday.

"Whether the uncertainties will be resolved, deepen or continue is something to be watched carefully."

Inter-Korean relations have been icy since Seoul in May accused Pyongyang of attacking a South Korean warship and killing 46 sailors, a charge the North has angrily denied.

The North has, however, recently made conciliatory gestures to the South.

It has also indicated willingness in principle to return to six-nation nuclear disarmament talks that it abandoned in April 2009, a month before its second atomic weapons test.

Campbell urged Pyongyang to mend ties with Seoul first and show it is serious about nuclear disarmament.

"The first step has to be a re-engagement between South Korea and North Korea. We are looking forward to a clear and demonstrable commitment on the part of North Koreans to fulfil commitments that they made on denuclearisation in 2005," he said, referring to an aid-for-disarmament pact that year.

The North's intentions remain unclear, with recent satellite photos suggesting it may be restoring facilities at the Yongbyon reactor which produced weapons-grade plutonium.

Pyongyang closed down the complex in 2007 under its disarmament deal, but announced after quitting the forum that it would restart operations there.

A senior Seoul presidential aide said this week the North's nuclear programme had reached a "very alarming level".

"If the nuclear warheads are made compact and deployed to the field, they could wreak immense havoc on South Korea regardless of their precision level," Kim Tae-Hyo, the president's deputy national security adviser, told a forum.