He died Saturday "from great mental and physical strain," during a train trip on "a field guidance tour," the Korean Central News Agency said.
Upon hearing of Kim's death, neighboring South Korea placed all its military units on an "emergency alert," its Yonhap News Agency reported. Tensions remain high on the Korean Peninsula since North Korea's nuclear and missile tests and subsequent maritime incidents.
South Korean President Lee Myung-bak canceled all his scheduled plans for Monday.
The White House in a statement Sunday night U.S. time said President Barack Obama had been notified of reports of Kim's death and that Washington was "in close touch" with its allies in South Korea and Japan.
"We remain committed to stability on the Korean Peninsula, and to the freedom and security of our allies," said the statement form the White House press secretary.
There have been reports that Kim's son Kim Jong Un, who is believed to in his 20s, was being groomed as a successor although questions remain whether he would be up to the job. The son already has been promoted to the rank of four-star general and holds major posts in the Workers Party.
Kim's death was also announced on state television by a female newscaster, clad in a black funeral dress.
"He suffered an advanced acute myocardial infarction, complicated by serious heart shock, on a train on Dec. 17 … from a great mental and physical strain caused by his uninterrupted field guidance tour for the building of a thriving nation," KCNA said.
Kim reportedly had not been in good health since suffering an apparent stroke in 2008.
China's Xinhua news agency quoted North Korean media as saying all possible first-aid measure was provided but Kim's life could not be saved. An autopsy Sunday had "fully confirmed the diagnosis of his diseases," the report said.
Kim Jong Il came to power in 1994 after the death of his father and national founder Kim Il Sung.
A Xinhua profile said Kim Jong Il was born on Feb. 16, 1942. He graduated in politics and economics from the Kim Il Sung University and began working for the Central Committee of the Workers' Party of Korea in 1964. He was elected party general secretary in 1997.
Yonhap, quoting KCNA, said Kim's body will be placed in the Kumsusan Memorial Palace where the embalmed body of his father lies. The funeral was set for Dec. 28 in Pyongyang and 13 days of state mourning was announced.
Kim's death comes at a time when the six nations involved in the North's nuclear disarmament are trying to resume their talks stalled the last three years after Pyongyang walked out. The parties to the talks include the two Koreas, Japan, China, Russia, and the United States.
Lately, the North had indicated its willingness to return to the talks. However, its relations with South Korea remains tense after an attack last year on a South Korean island and the sinking of a South Korean war ship.
Separately, Yonhap, quoting a senior South Korean Foreign Ministry official, said North Korea and the United States were likely to delay their bilateral nuclear talks set for this week in Beijing.
As for Kim's son succeeding him, Mike Chinoy, a senior fellow at the U.S.-China Institute, told CNN: "This has been in place for a while."
He, however, said without a figure like Kim Jong Il, it would be difficult to maintain unity and control given the country's economic and other problems.
Prof. Lee Jung-hoon at Yonsei University in Seoul told the BBC that while the succession issue remains to be completed, Kim's death could set off "very unstable times" in the country.
"We have to be very worried because whenever there is domestic instability North Korea likes to find an external situation to divert the attention away from that -- including indulging in provocation," he said.
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