By Ben Blanchard and Royston Chan

CHANGCHUN, China (Reuters) - A secretive convoy from North Korea reached the northeast Chinese city of Changchun on Saturday, in what may be the latest stop by the North's ruler Kim Jong-il as he seeks to shore up ties with his country's sole major supporter.

Neither Beijing nor Pyongyang has said whether he, or possibly his son and heir apparent Kim Jong-un, is visiting China. Both sides are habitually secretive about such trips, and there have been no definitive sightings.

But the tight security and unscheduled train movements echoed the script of past visits by 69-year-old Kim Jong-il, who visited twice last year to woo his powerful neighbor.

This latest mystery visit from the North comes as China's Premier Wen Jiabao prepares for a weekend summit in Tokyo with the leaders of Japan and South Korea, both foes of Pyongyang, which rattled the region last year and has drawn closer to Beijing for support.

Beijing has sought to steady ties with Seoul and Tokyo but also sees North Korea as a strategic buffer against the U.S. and its regional allies. In recent years, China has sought to shore up relations with the North with increased aid and trade and frequent visits there by leaders.

The arrival in China on Friday of a distinctive train from North Korea prompted South Korean officials and media to conclude it was carrying Kim Jong-un, anointed last year as heir apparent to his aging father, but speculation then shifted to it being the father.

On Saturday morning, the train reached Changchun, the capital of Jilin province in China's northeast, where the station's exit for official guests was cordoned off by police.

A convoy of a dozen or so cars which included a black limousine with darkened windows then took the mystery guests to the heavily guarded South Lake Hotel, accompanied by an ambulance, several mini buses and a police vehicles.

In the past, such visits have been shrouded in mystery, and China or North Korea have acknowledged a visit only near or after its end.

Kim Jong-il travels by train due to his fear of flying, and visited China last May and August. During his last trip, he met Chinese President Hu Jintao in Changchun and told him that Pyongyang remained committed to duclearization as per previous international agreements.

In November, however, the North showed a U.S. nuclear physicist what it said was a uranium enrichment programme, which could open a second route to make a nuclear bomb along with its plutonium programme.

That announcement, say Seoul, Tokyo and Washington, shows the North is not serious about its push to restart aid-for-disarmament nuclear talks, stalled for over two years.

Kim Jong-il is widely believed by South Korean officials and experts to have suffered a stroke in 2008, and analysts had thought his visits last year were aimed at shoring up support for a handover of power to his youngest son, Jong-un.

But the elder Kim's health appears to have improved significantly since the visits to China last year, prompting diplomats and analysts to re-evaluate their assessment of the pace of succession.

Recently published images of the so-called "Dear Leader" show him looking portly and well-fed, a far cry from the sickly figure photographed in parliament in 2009.

(Additional reporting by Maxim Duncan in HARBIN, China; Hyunjoo Jin in SEOUL; Additional reporting and writing by Chris Buckley in BEIJING; Editing by Sanjeev Miglani)