By Maxim Duncan

JILIN | Thu Aug 26, 2010 11:05am EDT

JILIN China (Reuters) -

North Korea's reclusive leader Kim Jong-il is visiting powerful ally China possibly with his son and heir apparent, South Korean government sources said on Thursday, ahead of a meeting that may settle Kim's succession.

South Korean officials have said Kim appears to be visiting northeast China. There have been no iron-clad sightings of the paunchy, frizzle-haired 68-year-old, but a hotel in the northeast city of Jilin was under heavy police guard on Thursday night, blocked off to reporters and ordinary residents -- a practice seen in Kim's past visits and perhaps a sign he is staying there.

Kim may be seeking China's acceptance of succession plans.

The Workers' Party (WPK), which rubber stamps major policy decisions in the secretive North, is holding a rare meeting in September at which the assembly could set in motion the succession of the leader's son, Kim Jong-un, analysts say.

"Kim Jong-il is traveling through China by train, but we have no information as to whether his son is accompanying him," a presidential source told Reuters.

A South Korean foreign ministry source said there was evidence that Kim and his son were in China. The South's Yonhap news agency reported Kim was in Jilin province, which has Jilin city as its capital, and he visited a school his father, the founding leader of North Korea, Kim Il-sung, studied at.

Kim, his iron rule underpinned by a personality cult, rarely travels abroad. But this would be the second time since May that he has gone to China on which he depends to prop up his country's failing economy.

When he does travel, he always goes by private train and is thought to be terrified of flying.

There is widespread speculation that Kim is in poor health following a suspected stroke in 2008 and some analysts say he may be in a hurry to establish his son's succession to the family dynasty that has ruled North Korea since its founding after World War Two.

PARTY MEETING

Daniel Pinkston, a specialist on Korean affairs in Seoul with the International Crisis Group, said a visit was most likely connected to next month's WPK meeting.

"There is so much circumstantial evidence pointing to the succession issue. And there are other signs that they are hurting for cash aid and assistance. The two things are not mutually exclusive.

"If the succession is being accelerated, then of course Kim has an incentive to address the economic problems and other issues which will be helpful for his son in the transition to taking power."

Wu Dawei, China's top envoy on stalled North Korean nuclear disarmament talks, told reporters at Seoul's Incheon airport he could not comment on the issue, but hinted at a visit.

"What I would like to stress is that China and North Korea are close neighbors and it is a normal thing for leaders of the two countries to exchange visits frequently," he said, after arriving in South Korea for talks on nuclear disarmament.

A policewoman at the Chinese border town of Ji'an said: "Some leader came yesterday," but declined further comment on reports that the two Kims had traveled there on late on Wednesday.

Cai Jian, an expert on Korea at Fudan University in Shanghai, expressed a degree of skepticism over the reported trip.

"North Korea does not need China's blessing or approval for succession plans, but Kim may feel he should inform China of his plans.

"China has made it clear that it is very concerned about maintaining stability and close contacts with North Korea, and it would be concerned to know that any future leader also attaches much importance to relations with China. But this is not about seeking China's agreement, it's about informing China."

The reported visit was taking place a day after former U.S. President Jimmy Carter flew into Pyongyang to win the release of an American jailed in the isolated country.

There has been heightened tensions on the peninsula after the March torpedoing of a South Korean warship, blamed by Seoul on the North. The sinking prompted expanded U.S. sanctions against the North. Pyongyang denies sinking the ship.

China wants Seoul and Washington to put the sinking of the warship behind them, and to restart the stalled six-party talks aimed at ending the North's nuclear weapons program.

(Additional reporting by Jeremy Laurence, Brett Cole and Kwon Youri in Seoul, Tabassum Zakaria in Washington and Chris Buckley in Beijing; writing by Chris Buckley and Jonathan Thatcher; Editing by Sanjeev Miglani)