The announcement was published in state media hours before a historic Workers' Party meeting Tuesday where Kim, 68 and apparently in deteriorating health, was expected to grant son Kim Jong Un and other family members top posts in plans to take the communist dynasty into a third generation.
The North Korean capital was in a festive mood, with banners and placards celebrating the country's biggest political gathering in 30 years. Kim Jong Il was re-elected to the party's top position of general secretary, but state media gave no further details on what was discussed or decided.
Footage shot by broadcaster APTN later Tuesday showed buses carrying party delegates leaving Pyongyang, a sign the gathering may have ended.
The succession within the opaque regime is of regional significance because of Pyongyang's nuclear and missile programs and continuing tensions between North Korea and rival South Korea - where the U.S. retains a major military presence.
It was the North Korean state media's first mention of Kim Jong Un, who has remained so well hidden from the outside world that not even his face or exact age can be confirmed. He is believed to be 27 or 28, and is said to have been schooled in Switzerland and educated at Kim Il Sung Military University in Pyongyang.
However, it is clear that "Kim Jong Un's promotion is the starting point for his formal succession to power," said Kim Yong-hyun, a North Korea expert at Seoul's Dongguk University.
"It's clearly the biggest news we've had from North Korea since the death of Kim Il Sung," said Peter Beck, a Council on Foreign Relations-Hitachi research fellow at Keio University in Tokyo.
"I think it clearly demonstrates that Kim Jong Il is committed to maintaining control of the country within his family," he said.
The appointment also appears aimed at putting the son at the helm of his father's "songun," or military-first, policy. He is expected to take up other top military jobs such as commander of the 1.2 million-member military, analyst Cheong Seong-chang of the private Sejong Institute think tank said.
The secrecy surrounding the succession process is typical of North Korea, and reminiscent of Kim Jong Il's own rise to power.
Kim Jong Il was 31 when he won the No. 2 post in the ruling Workers' Party in 1973, an appointment seen as a key step in the path to succeeding his father, North Korea founder Kim Il Sung.
The following year, Kim was formally tapped as the future leader but state media did not reveal that to the outside world until the landmark 1980 convention, the last big political meeting in North Korea.
He took over as leader in 1994 when his father died of heart failure in what was communism's first hereditary succession.
Who will become the North's next leader is being closely watched in other nations.
In Tokyo, Foreign Minister Seiji Maehara told reporters Tuesday that Japan's goal to end Pyongyang's nuclear program remains unchanged whatever change is made in North Korea's leadership. U.S. Assistant Secretary of State Kurt M. Campbell said in a conference call that Washington is "watching developments carefully" for clues to North Korea's nuclear policy.
In Seoul, South Korea's ruling Grand National Party denounced the North's succession process, calling it "anachronistic."
It's not known what kind of party position Kim Jong Un might be granted. Some predict he'll win the same post his father took 37 years ago: party secretary authorized to supervise party members and appoint top party, government and military officials.
However, the younger Kim may not have the benefit of 20 years of training like his father.
Kim Jong Il, said to be suffering from diabetes and a kidney ailment, reportedly had a stroke in August 2008, sparking fears about instability and a possible power struggle in the nuclear-armed country if he were to die without anointing a successor.
Kim has two other sons but the youngest is said to be his favorite.
However, with Kim Jong Un still in his 20s and politically inexperienced, Kim Jong Il may tap his sister to oversee an eventually transfer of power, experts say.
Sister Kim Kyong Hui, 64, was among six people promoted to general along with Kim Jong Un, and her name was listed ahead of Kim Jong Un's in the Korean Central News Agency report.
"There is a possibility that she could play the role of a coordinator to make sure the power succession goes smoothly," analyst Cheong said.
She and her husband, Jang Song Thaek, appointed a vice chairman of the all-powerful National Defense Commission in June, also are expected to win key political appointments that would allow them to act as advisers to the young Kim during his rise to power.
"The key here is bloodline," Ha Tae-keung, chief of Open Radio for North Korea, a Seoul-based radio station specializing in North Korean affairs, told reporters in Seoul. "Both Jang and his wife will never rule North Korea as Jong Un's regents, and will only function as advisers."
In a brief announcement Tuesday, a North Korean news reader said "crucial developments" were taking place at the political convention under way in Pyongyang and that Kim Jong Il was re-elected to the party's top position of general secretary to delegates' cheers of "Hurrah."
"His re-election is an expression of absolute support and trust of all the party members, the servicepersons and the people in Kim Jong Il," KCNA said, calling it a "historic" event.
The convention, initially set for early September, appears to have been delayed by several weeks amid speculation that Kim's health or damage from flooding and typhoons may have forced the postponement.
"This meeting of the delegates is an important occasion for further strengthening the solidarity of our army, our party, and our people, who are rallying behind the great Gen. Kim Jong Il," Kim Chang Gyong, an assistant professor at North Korea's Academy of Social Sciences, told broadcaster APTN.
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Associated Press writers Kelly Olsen, Kwang-tae Kim, Jean H. Lee and Sangwon Yoon in Seoul and Mari Yamaguchi in Tokyo contributed to this report.
