North Korean leader Kim Jong-Il was believed to be visiting key ally China on Thursday, possibly with his youngest son and presumed successor, officials, media reports and Chinese residents said.

The apparent trip, which was not confirmed by either Beijing or Pyongyang, dashed hopes of a meeting with former US president Jimmy Carter who is on a mission to North Korea to try to win the release of a jailed American.

"Judging from circumstances, Chairman Kim might have left for China early Thursday morning," a senior South Korean official told AFP on condition of anonymity.

Residents of Jilin in northeastern China said a delegation had visited the city Thursday amid tight security.

The trip -- which would be Kim's second to China this year -- comes amid increasing speculation about Kim's successor and efforts by Beijing to revive North Korean nuclear disarmament talks despite high tensions on the peninsula.

Analysts in South Korea said Kim was seeking to obtain China's blessing for his successor, widely expected to be his youngest son Kim Jong-Un, and gain desperately needed economic assistance from its main source of aid.

They also suggested that Kim's departure for China while Carter was in Pyongyang meant the North considered it too early to seek a breakthrough in tense relations with the United States.

South Korea's Yonhap news agency said Kim might be accompanied by Kim Jong-Un, 27, who is expected to be named to the ruling North Korean party leadership at a rare meeting next month.

The meeting would represent "a landmark of an epochal turn in strengthening the party," the North's ruling party newspaper Rodong Sinmun said Thursday.

Speculation about the succession has intensified since Kim Jong-Il, now 68, suffered a stroke in August 2008, but he has since recovered sufficiently to work.

"Kim may have decided to tackle this issue in person as China has yet to reach an understanding about the succession at a time when the nuclear issue has not yet been resolved," Professor Yang Moo-Jin of the University of North Korean Studies told AFP.

Yonhap quoted an official at South Korea's presidential office as saying that Kim crossed into China at about midnight Wednesday on his personal train.

In Jilin, a resident said the group visited a school Thursday that had been attended by Kim's father and former North Korean leader Kim Il-Sung as a boy.

"They arrived in the morning. There were many police in the streets and the roads were blocked," a woman who works at a restaurant adjacent to the Yuwen Middle School told AFP by phone.

Yonhap quoted a diplomatic source in Beijing as saying the delegation was expected to stay at the five-star Crystal Hotel in Jilin. It was unclear whether they would travel to Beijing.

Pyongyang and Beijing have made it a rule not to confirm Kim's trips to China, which he last visited in May and met President Hu Jintao.

Chung Seong-Chang of the Sejong Institute said Kim's visit underscored Pyongyang's desperate need for food aid and construction materials from China.

"Ahead of the party convention, Kim has to ease complaints over food shortages and stabilise people's livelihood," he said.

North Korean state media said Beijing has offered emergency aid following flooding which washed away homes, roads, railways and farmland in North Korea, causing an unknown number of deaths.

Beijing has also been stepping up efforts to resume six-party talks on disarming the North amid simmering tensions over the sinking of a South Korean warship in March, for which Pyongyang was blamed.

China's top nuclear envoy Wu Dawei met his South Korean counterpart Wi Sun-Lac on Thursday after travelling to Pyongyang last week, but said it was too early to discuss the outcome.

Wu said at the meeting that the North had reiterated its willingness to start preliminary talks, according to Yonhap, which cited a South Korean foreign ministry official.

Kim's departure means he is not likely to meet Carter, who was nevertheless treated as a VIP by Pyongyang, which staged a warm official welcoming ceremony for the former US president.

Carter held a "cordial" meeting with the North's number two Kim Yong-Nam on Wednesday, KCNA said.

The Nobel peace laureate arrived on Wednesday in a bid to secure the release Aijalon Mahli Gomes, an African-American who was jailed in April for illegally crossing into the North from China.