North Korean leader Kim Jong-Il has died aged 69 of a heart attack, state media announced Monday, plunging the impoverished but nuclear-armed nation into uncertainty as it faced a second dynastic succession.

The leader "passed away from a great mental and physical strain" at 8:30 am on Saturday (2330 GMT Friday), while travelling by train on one of his field tours, the official Korean Central News Agency (KCNA) said.

It urged people to follow his youngest son and heir apparent Kim Jong-Un, who is aged in his late 20s and was last year made a four-star general and given top ruling party posts despite having had no public profile.

"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-Un and protect and further strengthen the unified front of the party, military and the public," said a weeping black-clad TV announcer.

KCNA said Kim died of a "severe myocardial infarction along with a heart attack" and that an autopsy was performed Sunday.

The leader suffered a stroke in August 2008 which left him with impaired movement in his left arm and leg, and triggered an acceleration in the succession plans.

Kim's funeral will be held on December 28 in Pyongyang but no foreign delegations will be invited, KCNA said. A period of national mourning was declared from December 17 to 29.

"We must hold high the flag of songun (military-first) policy, strengthen military power a hundred times and firmly defend our socialist system and achievement of revolution," KCNA said.

At the North Korean embassy in Beijing, the capital of its main ally China, the national flag was flying at half mast.

North Korea's propaganda machine has rolled into action to build up the same personality cult for Jong-Un that surrounded his father and late grandfather Kim Il-Sung, the founder and "eternal leader" of North Korea who died in 1994.

However, little is known about the succession. South Korea's top official on cross-border affairs said last month that there would be challenges in transferring power to the son.

Kim Jong-Il's only sister Kim Kyong-Hui and her husband Jang Song-Thaek, the country's unofficial number-two leader, are expected to act as the son's guardian and throw their political weight behind him, analysts say.

The news sent shockwaves around the region where for years tensions have run high over North Korea's nuclear ambitions and aggressive tactics.

South Korea placed its troops and police force on emergency alert, and summoned a meeting of the National Security Council. President Lee Myung-Bak cancelled his schedule and called an emergency cabinet meeting for 3 pm.

The Joint Chiefs of Staff said it had increased monitoring along the border along with US forces in the country but that no unusual activity had been observed.

North and South Korea have remained technically at war since their three-year Korean conflict ended only in an armistice in 1953.

The news shocked South Koreans and some expressed fears of renewed conflict.

"I'm worried there will be a war. I thought it wasn't true at first," said student Song Bo-Na, 22.

The United States, which stations 28,500 troops in the South, said it was closely monitoring events.

"The President has been notified and we are in close touch with our allies in South Korea and Japan," said White House spokesman Jay Carney.

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

Japan, which has no diplomatic relations with North Korea, offered its condolences over the death.

The Japanese authorities also called an emergency security meeting and, minutes after the noon broadcast by Pyongyang, Prime Minister Yoshihiko Noda cancelled a speech and rushed to confer with senior ministers.

Australia said it was critical that governments including North Korea's exercised calm and restraint after the death, and urged Pyongyang to engage with the global community.

KCNA, quoting a statement from the national funeral committee headed by Jong-Un, said Kim Jong-Il's body would lie in state in Kumsusan palace where his father's embalmed body is on display.

It said mourners would be allowed to visit from December 20 to 27.

Following the funeral, another event to mourn the leader would be held on December 29. Mourning shots are to be fired and three minutes of silence would be observed. All trains and ships will sound their horns.

Kim took over after his father and founding president Kim Il-Sung died in 1994.

In the mid- to late-1990s he presided over a famine which killed hundreds of thousands of his people. Severe food shortages continue and the UN children's fund estimates one-third of children are stunted by malnutrition.

But Kim still found resources to continue a nuclear weapons programme which culminated in tests in October 2006 and May 2009. The country is believed to have a plutonium stockpile big enough for six to eight weapons.

For several months there have been diplomatic efforts to restart six-nation nuclear disarmament talks which the North abandoned in April 2009.

US envoy Robert King held talks in Beijing last week about the possible resumption of US food aid. There had been speculation the two sides would meet in Beijing this week for separate talks about reviving the six-party process.