North Korea's state news agency KCNA has called Kim Jong-un, the late leader Kim Jong-il's son, a "great successor" shortly after the news about the dictator's death.

The "dear leader," who has led the secretive nuclear-armed state since 1994, died aged 69 while traveling on his train on Saturday.

Kim Jong-il died of a heart attack due to "physical and mental over-work," North Korean state television announced, as cited by RIA Novosti.

KCNA urged the communist nation to rally behind the younger Kim, who is believed to have been groomed to take over power and was appointed a general last year.

The dear leader's enigmatic third son Kim Jong-un, who is thought to be in his late 20s, is the "great successor to the revolutionary cause of Juche [North Korea's national ideology of self-reliance] and outstanding leader of our party, army and people," KCNA said.

Global media remind that under late dictator Kim Jong-il North Korea has gone through a prolonged spell of economic hardship and food shortages, with some suggesting that a humanitarian disaster is unfolding in the reclusive state.

A funeral for Kim Jong-il will be held in Pyongyang on December 28, with the mourning period lasting until December 29, KCNA said.

No foreign delegations will be allowed to attend the funeral. Kim's body is now laid in a massive mausoleum in Pyongyang where his father, Kim Il-sung, lies in state in a glass case.

News of Kim's death came in an emotional bulletin broadcast on state television.

"It is our people and nation's biggest sadness," the tearful announcer, wearing black, said as she told viewers of the dear leader's death.

The US White House spokesman Jay Carney said President Barack Obama had been notified and that Washington was "in close touch with our allies in South Korea and Japan."

"We remain committed to stability on the Korean peninsula, and to the freedom and security of our allies," Carney said.

A spokesman for South Korean President Lee Myung-Bak said the leader spoke to Obama over the telephone and that the two agreed to "closely cooperate and monitor the situation together," the BBC reported.

China, Pyongyang's closest ally, has yet to officially comment. Asian stock markets slumped after Kim's death was announced.