Kim Jong-Il's secretive visit to China made it clearer than ever that the ageing North Korean leader is preparing an eventual power transfer to his youngest son Jong-Un, analysts said Tuesday.

The two nations were also staging a show of unity as Pyongyang faces tougher sanctions for its alleged torpedo attack on a South Korean warship, they said.

Kim, 68, stressed the need to prepare for the "rising generation" and visited sites linked to his own late father and founding president Kim Il-Sung during a five-day trip that ended Monday.

Analysts saw the site visits as a bid to confer legitimacy on another dynastic succession in the hardline communist state.

The Swiss-educated Jong-Un, believed aged around 27, could be named to a senior post when the North next month holds its first meeting for decades of key ruling party delegates.

Chinese President Hu Jintao, at a welcoming banquet last Friday, reportedly wished "a signal success" for the meeting.

"Depending on Kim Jong-Il's health, the son will possibly be appointed as party organisation secretary or as organisation department head just below the (secretary) position," Paik Hak-Soon of Seoul's Sejong Institute told AFP.

Yang Moo-Jin of Seoul's University of North Korean Studies said Kim and Hu "must have discussed the succession issue deeply".

Cheong Seong-Chang, also from the Sejong Institute, said Kim must have notified key ally China of upcoming leadership changes but did not seek consent.

"North Korea, as a proud sovereign state, needs no approval for its leadership changes from any country," Cheong told AFP. "After being informed in advance of the upcoming changes, China must have felt satisfied.

"Jong-Un has already been in charge of personnel appointments in the party. Since the second half of last year, all reports to the leader have been made through Jong-Un."

Cheong said the son is likely to be appointed party organisation secretary in charge of filling party posts, making policy decisions and managing reports presented to the leader.

Kim's visit to China -- his second since May -- was also designed to underscore their traditional alliance amid rising regional tensions, analysts said.

"They are stepping up the alliance and demonstrating it towards the United States and its allies amid an intensifying enmity between the two great powers in the region," Paik said.

"China was sending a strong message to the United States, Japan and South Korea -- 'Don't shake the North'."

Tensions have risen sharply since the United States and South Korea accused the North in May of sinking a South Korean warship -- a charge it denies. A subsequent US-South Korean naval exercise strained ties between Washington and Beijing.

Washington on Monday slapped new sanctions on four North Korean people and eight organisations.

China's state-controlled Global Times Tuesday called on the United States, Japan and South Korea not to "bully" the North any more if they want to ease regional tensions.

"Beijing is expanding its influence in the region as the North finds no one else to turn to but China, as South Korea has been cutting off economic aid to the North and the United States announces new sanctions," Yang said.

Chinese TV footage released Monday also showed Hu urging Kim to push forward economic reforms and modernisation in North Korea.

According to China's Xinhua news agency, Hu said "economic development calls for self-dependence but cannot be achieved without cooperating with the outside world".

Despite severe food shortages Kim's regime has resisted Chinese-style reforms for fear of weakening its grip over the economy.

Kim expressed hope for an early resumption of six-party nuclear disarmament talks, according to Chinese media, although the North's news agency made no mention of this.

China hosts the talks which the North quit in April 2009. It staged its second atomic weapons test a month later.

Using economic aid and cooperation as leverage, China will persuade the North to permit the return of International Atomic Energy Agency inspectors, Yang predicted.