By Ben Blanchard and Royston Chan

CHANGCHUN, China (Reuters) - A train from North Korea headed through China on Saturday on secretive journey that may be a mission by leader Kim Jong-il's to shore up ties with Pyongyang's sole major supporter.

Neither Beijing nor Pyongyang has said whether Kim, and maybe his son and heir apparent Kim Jong-un, is visiting China on the distinctive North Korean train. Both sides are habitually secretive about such trips.

But the tight security and unscheduled route of the train echoed past visits by the elder Kim, who visited China twice last year to woo his powerful neighbor.

On Saturday, the delegation rolled into Changchun, capital of Jilin province. A convoy of cars that included a black Mercedes Benz with darkened windows took mystery guests into a heavily guarded hotel.

Later the motorcade left for the train station, and the visitors continued their rail journey, passed through Shenyang, the capital of Liaoning province, and in the evening headed in the direction of the national capital, Beijing, according to South Korea's Yonhap news agency.

There was no certainty, however, that the train would stop in Beijing. Kim's movements are always cloaked in secrecy, and in the past, China and North Korea have acknowledged his visits only near or after their end.

Japan's Kyodo news agency issued a photograph that appeared to show the 69-year-old Kim Jong-il, with his distinctive frizz of hair and olive-green clothing, leaving a hotel in Mudanjiang, where he apparently stopped on Friday before going to Changchun.

China's Premier Wen Jiabao is currently visiting Japan for a summit with the leaders of Japan and South Korea, both foes of Pyongyang.

North Korea rattled the region last year with its increasingly belligerent stance toward the far richer South, and has drawn closer to neighbor Beijing for economic and diplomatic support.

STRATEGIC BULWARK

China has sought to steady ties with Seoul and Tokyo, but also sees North Korea as a strategic bulwark against the United States 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 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 father, but speculation then shifted to it being the father.

A South Korean newspaper, the Chosun Ilbo, reported that Kim Jong-un, in his late 20s, was not among the North Korean visitors, citing government sources.

Kim Jong-il travels by train due to his fear of flying, and visited China last May and August. Northeast China could play an important part in North Korea's economy, and Kim Jong-il has traveled there before, including last year when Chinese President Hu prodded him to open up the North's economy.

During that trip, Kim told Hu that Pyongyang remained committed to dismantling nuclear facilities in line with previous international agreements.

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

Kim Jong-il is widely believed by South Korean officials and experts to have suffered a stroke in 2008. But his health appears to have improved since the visits to China last year, prompting diplomats and analysts to re-evaluate their assessment of the pace of succession.

(Additional reporting by Maxim Duncan in CHANGCHUN, China; Hyunjoo Jin in SEOUL; Additional reporting and writing by Chris Buckley in BEIJING; Editing by Angus MacSwan)