North Korea's leader-in-waiting Kim Jong-Un began a visit to China Friday, South Korean media reported, a trip seen as conferring Beijing's blessing on its close ally's succession process.

The trip would be the first since the young Jong-Un was given key military and political posts last September, clarifying his status as heir apparent to his ageing father Kim Jong-Il.

China is the impoverished North's sole major ally and its economic prop. Its support for an eventual succession is crucial as the North grapples with food shortages and international isolation over its nuclear and missile programmes.

"Kim Jong-Un arrived in China early this morning via Tumen," Yonhap news agency quoted a source in the northeastern Chinese border city as saying.

KBS TV said Kim's special train was also expected to stop at the cities of Mudanjiang, Harbin and Changchun, reversing the route his father took when he visited northeast China last August.

"I saw a lot of police in uniform at the (Tumen) station and then I saw a train marked by a North Korean flag slowly passing by," a blogger said on China's Baidu site.

"Later, I learned that Kim Jong-Un's train had gone through."

An unidentified senior South Korean official told Yonhap it was unclear whether both father and son were making the visit "but it is more likely that this is a solo visit by the son. Apparently, his destination is not Beijing."

YTN television said the son was accompanied by his uncle Jang Song-Thaek, who wields great political influence and is expected to act as "regent" if the son, now aged about 27, takes power sooner than expected.

Kim, now 69, apparently speeded up succession plans after suffering a stroke in August 2008. Last September his youngest son was appointed a four-star general and given senior posts in the ruling communist party.

Since then the chubby Swiss-educated youth has accompanied his father on a variety of inspection visits and appeared at public events.

"This visit to China, the sponsor state of the North, will help stabilise the North and cement the father-to-son succession system," Professor Yang Moo-Jin of Seoul's University of North Korean Studies told AFP.

"This will also be helpful for China in strengthening its growing influence over North Korea, especially in terms of economic cooperation."

Kim Jong-Il took power after his own father and founding president Kim Il-Sung died in 1994.

"This (trip) is aimed at solidifying Jong-Un's status as the heir-apparent," a Seoul government official told Yonhap. The news agency said there was speculation Jong-Un may meet Vice President Xi Jinping, tipped as China's future leader, in Changchun.

Previous trips by Kim Jong-Il to China have been shrouded in secrecy, with state-controlled media in both countries reporting them only after they end.

China's economic influence over its neighbour has grown as South Korea and Western nations cut ties amid tensions over the North's nuclear and other military programmes.

The North's trade with China rose 32 percent last year to $3.47 billion, after South Korea severed some trade ties in protest at border attacks blamed on its neighbour.

China's President Hu Jintao urged Kim senior to open up the North's state-directed economy when they met last August in Changchun.

The North is seen as hesitant to adopt Chinese-style economic reforms for fear of loosening the regime's grip over its people.