Kim Jong-Il apparently died of a heart attack, brought on by stress and overwork, while travelling on a train. A spokesperson for Barack Obama said that the White House has been notified of his death.
His funeral is planned for December 28 in Pyongyang, with a mourning period to last until December 29.
Ruthless leader Kim Jong-Il came to power in 1994, succeeding his father, Kim II Sung. If Jong Un takes control of North Korea, he will be the third ruler in the family dynasty.
South Korea and Japan have put their militaries on high alert over fears that power struggles taking place behind the scenes could erupt as politicians clash with the military over the country’s future.
The U.S has said it may now postpone decisions on renewing talks on nuclear power with North Korea.
Although there was much public grief after news of Kim Jong-Il’s death, experts on North Korea say that there is growing discontent within the country.
Severe food shortages have left North Korea badly in need of food aid and the UN children’s fund estimates one-third of children are stunted by malnutrition.
It is thought that 200,000 are currently in prison camps.
North Korea has continued working on a controversial nuclear weapons programme and is believed to have a plutonium stockpiles big enough for six to eight weapons.
However, according to Associated Press, residents of Pyongyang burst into tears on the street after hearing of Kim Jong-Il’s death.
“It is the biggest loss for the party … and it is our people and nation’s biggest sadness,” a tearful anchorwoman clad in black Korean traditional dress told viewers as she announced Kim’s death.