North Korean leader Kim Jong-il visited China early Thursday, throwing another curveball a day after former U.S. president Jimmy Carter arrived in Pyongyang.

“We have found evidence of Kim’s train crossing the border around midnight from North Korea’s Manpo to Jian (in China’s Jilin Province),” a presidential aide said on condition of anonymity.

“It is different from Kim’s usual route through Dandong. He has never visited China twice over such a short period of time, so we are taking caution to figure out the purpose of his trip.”

Kim arrived in the city of Jilin Thursday afternoon where he visited an alma mater of his father Kim Il-sung and Beishan Park where China’s communist leaders are buried, according to sources here.

Kim Jong-il held a summit meeting with Chinese President Hu Jintao in Beijing early May in his first reported visit in four years to the neighboring country.

Some North Korea watchers suspect that Kim’s third son and successor Jong-eun is likely to be accompanying his father to China ahead of the Workers’ Party meeting of top representatives early next month, where the 26-year-old is expected to be named to key government posts.

Diplomatic sources in Seoul note the possibility of Kim Jong-il traveling to Beijing for talks with China’s top leadership as well as to other parts of the country.

Kim’s excursion across the border threw cold water on former U.S. president Jimmy Carter’s visit to Pyongyang. Carter arrived Wednesday on a mission to rescue an American citizen sentenced to eight years of hard labor there on charges of illegal entry.

“Kim appears to have shown that it will be different to the time Bill Clinton visited Pyongyang a year ago, while underlining the North’s strong ties with China over its relations with the U.S.,” said Yu Ho-yeol, professor of North Korean studies at Korea University.

“It is possible Kim may have taken Jong-eun with him to China ahead of the Workers’ Party meeting of top representatives” from Sept. 6-8.

Kim Jong-il, who was formally named the successor to his father Kim Il-sung at the party congress in 1980, was invited to China three years later by then Chinese leader Deng Xiaoping.

Kim has a number of reasons to visit the North’s closest ally and main provider of economic aid especially as diplomatic efforts are underway to resume the stalled six-nation negotiations.

“In addition to his power transfer, Kim could discuss with the Chinese leadership the circumstances for the six-party talks, economic aid and how to ease tension on the Korean peninsula,” Yu said.

China’s chief nuclear envoy Wu Dawei traveled to Pyongyang last week and told Japanese reporters that the North agreed to hold preliminary talks before a formal round of the six-party talks. The North’s state media had reported the two allies reached a “full consensus” on matters of mutual concern during Wu’s visit.

Wu arrived in Seoul Thursday to discuss the circumstances for the multilateral talks with his South Korean officials.

Seoul and Washington have urged the North to rejoin the aid-for-denuclearization talks without any preconditions, but had called on Pyongyang to apologize for its torpedo attack on the South Korean naval ship in March before joining the talks.

But a South Korean official said Wednesday that the six-nation negotiations and the Cheonan’s sinking were two different matters.

North Korea, suffering from a chronic food shortage and international sanctions, was recently hit by a major flood in the northwestern region along the Amnok (Yalu) River.

Following his previous trips in 2000, 2001, 2004, 2006 and May this year, it will be Kim’s sixth official visit to China.

By Kim So-hyun (sophie@heraldcorp.com)