North Korean dictator Kim Jong Il traveled to northeastern China—his second trip in four months to Pyongyang's chief ally—to visit a school attended by his father, in a symbolic move that appeared aimed at solidifying plans for his own succession.

Thursday's visit, disclosed by South Korean officials and confirmed by local Chinese residents informed of it, comes about two weeks ahead of a major political gathering in North Korea, where Mr. Kim is expected to appoint his third son, Kim Jong Eun, to a position that would publicly signal he will eventually become the country's next top leader. South Korean media reports said that Kim Jong Eun accompanied Mr. Kim to China Thursday, but that couldn't be confirmed.

Mr. Kim left North Korea apparently without meeting former U.S. President Jimmy Carter, who arrived Wednesday in Pyongyang to retrieve an American teacher who walked into North Korea from South Korea in January.

Mr. Kim's travel to China during Mr. Carter's visit is a sign that the succession issue is now at the top of the North Korean leader's agenda. North Korea's Workers Party will hold a meeting of party representatives next month, the first since 1966, and many analysts believe the gathering will be used to elect new leaders and even if Mr. Kim's son is appointed to an official post at the gathering, some say his rise is far from guaranteed. For Mr. Kim to embark on this trip to China now could be a way of invoking the Kim family mythology to attach some legitimacy to his son.

The son, Kim Jong Eun, has never been seen in public or in the media in North Korea. And Kim Jong Il's poor handling of the country's economy, most recently illustrated by severe shortages after a move to suppress market activities, has weakened support for his regime.

Mr. Kim visited Yuwen Middle School in the Chinese city of Jilin approximately 250 kilometers, or about 155 miles, from the border with North Korea, Jilin residents and neighbors of the school said. Students at the school were given a one-day holiday, the residents said. They were unsure if Mr. Kim's son was also there.

With the Jilin trip, Mr. Kim may be trying to remind ordinary North Koreans of the importance of his father—North Korea's founder Kim Il Sung, who led the country for 46 years until his death in 1994 and is still referred to as "eternal president"—and to build support for another generation of Kim family leadership.

Kim Il Sung attended the school as a teenager, and it is said to be the place where he first learned communist ideology. The story of Kim Il Sung's journey from Pyongyang to Jilin in 1925 is a staple in North Korean education, key to the mythology of him as someone who returned as a liberator of the country from Japanese colonial rule. People and groups who re-create the trip are occasionally mentioned in its state-run media.

It was unclear whether Mr. Kim planned to travel elsewhere in China. Chinese officials declined to comment on Mr. Kim's visit. Beijing hasn't confirmed past visits until after Mr. Kim's return to North Korea.

China is North Korea's chief economic benefactor and main political ally, but it is unclear what role Beijing is playing, if any, in the succession process.

While some analysts have pointed to signs of Chinese frustration toward Mr. Kim's regime, Beijing is certain to want to see an orderly transition of power to prevent chaos on its border.

Despite China's importance to North Korea, this trip is just Mr. Kim's sixth visit to the country as North Korean leader. A visit in May, which included stops in Dalian and Beijing, was his first since 2006. During that visit, Mr. Kim met with Chinese Communist Party chief Hu Jintao and other top leaders.

Because he fears planes, Mr. Kim typically travels by train and his movements are easily monitored by spy satellites used by South Korea, the U.S. and other countries. Word of his trip first came from the South Korean government, which announced early Thursday that it had detected signs of movement by the train used by Mr. Kim.

Mr. Kim's train passed through the Chinese border city of Ji'an Wednesday night, according to residents there. He didn't stop in the city. The area around the border crossing was blocked off by police earlier Wednesday, the residents said.

Last week, a former prime minister known as an economic reformer, Pak Pong Ju, attended a state function, raising speculation that Mr. Kim might again be reversing his approach after the clampdown and becoming interested in more open economic behavior.

Mr. Kim's decision to miss Mr. Carter comes as something of a surprise. In August 2009, former U.S. President Bill Clinton went on a similar mission and brought out American journalists Laura Ling and Euna Lee, who were detained by North Korean soldiers on the Chinese border in March 2009.

Mr. Kim met Mr. Clinton at that time and, the following day, North Korean newspapers were filled with pictures of the two men. During Mr. Carter's visit, North Korean state media have released photos of the former U.S. president meeting with Kim Yong Nam, the nominal No. 2 leader of the country.