Source: Deutsche Welle
The veteran leader died on December 17 at 8:30 local time on a train trip, a weeping announcer said, saying that he had died of fatigue.
The news came in a "special broadcast" by Pyongyang early on Monday. It had previously been expected that the announcement would relate to media reports in South Korea that the North has agreed to suspend its uranium enrichment program.
A few hours after the announcement of Kim Jong Il's death, North Korea's official state news agency KCNA declared his youngest son, Kim Jong un as the "great successor to the revolutionary cause of Juche and outstanding leader of our party, army and people."
Juche is Pyongyang's political ideology of self-reliance.
Kim Jong Un had been seen as the leader-in-waiting after being appointed to a senior military post and political post in 2010.
South Korea's military was placed on high alert Monday, but said it had detected no signs of unusual movement by the North's military following news of the death.
South Korean President Lee Myung-bak discussed the situation in a telephone conversation with his U.S. counterpart Barack Obama.
"The president reaffirmed the U.S.' strong commitment to the stability of the Korean peninsula and the security of our close ally, the Republic of Korea," the White House said in a statement.
"The two leaders agreed to stay in close touch as the situation develops and agreed they would direct their national security teams to continue close coordination."
Kim Jong Il is believed to have suffered a stroke in 2008. However, he had appeared relatively healthy in recent photos and video from recent trips to China and Russia.
The man known as the "Dear Leader" in North Korea is thought to have had a taste for cigars, cognac and gourmet cuisine. Some reports claim he had diabetes and heart disease.
He came to power following the sudden death of his father, the "Great Leader," Kim Il Sung in 1994.
KCNA reported that a period of national morning had been declared to run until December 29. Kim Jong Un is to head the committee to organize his father's funeral, which is to be held on December 28.