North Korean leader Kim Jong-Il visited the Chinese industrial city of Harbin on Sunday on a surprise trip to China believed linked to his country's succession process, a report said.

Kim arrived in the northeastern city at midnight and visited industrial sites and provincial officials there after prolonging his trip, South Korea's Yonhap news agency reported, citing sources in the city.

He was expected to leave Harbin -- the capital of Heilongjiang province -- at 10.00pm Sunday (1500 GMT) or 6.00am Monday (2300 GMT Sunday), as access to the train station will be restricted during those times, Yonhap said.

Kim is believed to have been in China since Thursday, travelling on his private train and possibly accompanied by his son Kim Jong-Un, his apparent successor as leader of the isolated communist state.

His train left the city of Changchun late Saturday, but on Sunday morning there were no signs it had returned to North Korea via the Chinese border cities of Dandong or Jian, Yonhap quoted a diplomatic source in Beijing as saying.

The agency also cited the northern Chinese towns of Yanji, Tumen and Hunchun as possible destinations for Kim's trip, which is his second to China in about three months even though he rarely travels abroad.

A source in Yanji told Yonhap the local government was preparing to receive a special guest. "It is highly likely that it would be Chairman Kim," the source said.

The South's cable news channel YTN meanwhile reported that work to clean up main streets and control traffic in Tumen began Sunday morning. It said 10 large sedans have been waiting in Namyang, a North Korean town bordering Tumen, since Saturday.

South Korean media has reported that Kim, 68, is believed to have met President Hu Jintao in Changchun, during an apparent mission to seek China's support for an eventual handover of power to his youngest son Jong-Un.

Yonhap quoted an employee of the city's Nanhu hotel as saying both leaders stayed in the establishment at the same time. Kim also met Hu during his previous visit in May.

Kim suffered a stroke in August 2008 and since then has speeded up plans for a power transfer in the hardline communist state.

China is the impoverished North's sole major ally and its economic lifeline. It chairs six-nation talks on the North's nuclear disarmament and has been pressing Pyongyang to return to the forum, which it quit in April 2009.

The latest trip went ahead despite last week's visit to Pyongyang by former US President Jimmy Carter to secure the release of a jailed American. It was not known whether Jong-Un, believed to be aged about 27, accompanied his father.

On the first day on Thursday, Kim visited Jilin's Yuwen Middle School, which his father, North Korea's founder Kim Il-Sung, attended from 1927 to 1930.

Yonhap said a visit by Kim to the Yanbian Korean Autonomous Prefecture, which contains the cities of Yanji, Tumen and Hunchun, may be related to economic cooperation plans.

Beijing is reportedly seeking permission to extend its lease of the North's northeastern port of Rajin, which provides access to the Sea of Japan (East Sea).

China currently has a 10-year lease on Rajin, which also borders Russia, Li Longxi, governor of the Yanbian prefecture, said during China's annual legislative meeting in March.

He said the use of the port would make it easier to ship coal from northeast China to southern China and Japan.

Official media in Beijing and Pyongyang have said nothing about Kim's visit, which may also be aimed at securing much-needed food and construction aid following recent severe floods.

Seoul officials said they would seek information from China as soon as Kim returns home.

"Customarily, China has briefed related countries about the result of Kim's visit once he returns home, and South Korea is certainly the number one priority in this matter," a foreign ministry spokesman told AFP.

"We expect the same process to take place very soon, possibly this week."