SEOUL (Reuters) - North Korean leader Kim Jong-il died of a heart attack, state media reported on Monday, sparking concern over who is in real control of the nuclear renegade state as his untested youngest son takes over the family dynasty.

A tearful television announcer, dressed in black and her voice quavering, said the 69-year old iron ruler died on Saturday of "physical and mental over-work" on a train on his way to give field guidance - advice dispensed by the "Dear Leader" on his trips to factories, farms and military bases.

North Korea's official KCNA news agency named Kim's youngest son, Kim Jong-un, as the "Great Successor", lauding him as "the outstanding leader of our party, army and people".

But there was uncertainty about how much support he has among the ruling elite, especially in the military, and worry he might try some military provocation to help establish his credentials.

"Kim Jong-un is a pale reflection of his father and grandfather. He has not had the decades of grooming and securing of a power base that Jong-il enjoyed before assuming control from his father," said Bruce Klingner, an Asia policy analyst at the Heritage Foundation in Washington.

"(He) may feel it necessary in the future to precipitate a crisis to prove his mettle to other senior leaders or deflect attention from the regime's failings."

Video from Chinese state television showed residents weeping in the North Korean capital Pyongyang. KCNA reported people were "writhing in pain" from the loss of the man who in 1994 assumed the leadership of the totalitarian state from his father Kim Il-sung, the North's first, and officially eternal, president.

News of the death of the man whose push to build a nuclear arsenal left the North heavily sanctioned and internationally isolated, triggered immediate nervousness in the region, with South Korea stepping up its military alert.

China, the North's neighbor and only powerful ally, said it was confident the North would remain united and that the two countries would maintain their relationship.

"We were distressed to learn of the unfortunate passing of (Kim) ... and we express our grief about this and extend our condolences to the people of North Korea," Foreign Ministry spokesman Ma Zhaoxu was quoted by Xinhua news agency as saying.

"We are confident the North Korean people will be able to turn their anguish into strength and unify as one," he said.

While his father had 20 years as official heir, Kim Jong-un only became successor in the period after Kim Jong-il suffered a stroke around August 2008.

He takes over a hermit state whose economy has been ravaged by decades of mismanagement under Kim Jong-il, who only briefly flirted with economic reform, preferring to stick with central planning and the brutal crushing of any opposition.

Under Kim Jong-il's rule, an estimated 1 million North Koreans died during famine in the 1990s. Even with good harvests, the state cannot feed its 25 million people.

Little is known of Jong-un, who is believed to be in his late 20s, studied for a short time at a school in Switzerland, and was appointed to senior political and military posts only last year.

KCNA said Kim Jong-il died at 8:30 a.m. on Saturday (2330 GMT on Friday) after "an advanced acute myocardial infarction, complicated with a serious heart shock".

South Korea, still technically at war with the North, placed its troops and all government workers on emergency alert, but said there were no signs of any unusual North Korean troop movements.

President Lee Myung-bak spoke on the phone with U.S. President Barack Obama.

The White House said Washington was committed to stability on the Korean peninsula as well as to its allies. There are some 28,000 U.S. troops on the divided peninsula. Across the heavily armed border, the North maintains an estimated 1 million troops, one of the world's largest standing armies.

Japan, too, said it was watching developments closely.

"We hope this sudden event does not have an adverse effect on the peace and stability of the Korean peninsula," Chief Cabinet Secretary Osamu Fujimura told a news conference after a hastily called ministerial meeting on security.

SHARES FALL

The fear of what might happen next in North Korea unsettled financial markets, with Asian shares and U.S. index futures falling. South Korean stocks tumbled as much as 5 percent, and the U.S. dollar gained. The Korean won fell 1.8 percent.

Kim Jong-un was at the head of a long list of officials making up the funeral committee, indicating he would lead it, and a key sign that he had taken, or been given, charge.

But there will be questions over how much real control the younger Kim has, and whether the military elite accepts him.

Zhu Feng, Professor of International Relations at Peking University, said it was clear the mechanism for transition was in place and working.

"The issue of primary concern now is not whether North Korea will maintain political stability, but what will be the nature of the new political leadership, and what policies will it pursue at home and abroad.

"In the short-term, there won't be new policies, only a stressing of policy stability and continuity. So soon after Kim Jong-il has died, no leader will dare say that an alternative policy course is needed," Zhu said.

But Chung Young-Tae at the Korea Institute of National Unification said there was "a big possibility that a power struggle may happen."

UNCHALLENGED HEAD

Kim Jong-il also promoted his sister and her husband, Jang Song-thaek, to important political and military posts, creating a powerful triumvirate.

Chang is seen as effective regent for the younger Kim. He holds a top position in the powerful Worker's Party providing some balance to the generals who have been seen as more hardline in pushing the North to develop an atomic arsenal.

Earlier this decade, Chang was forced into exile for what is believed to have been conflict over his push for economic reforms.

Experts say Jong-un has the intelligence and leadership skills that make him suitable to succeed his father. He is also reported to have a ruthless streak that analysts say he would need to rule North Korea.

There is likely to be an outpouring of emotion over Kim's death in North Korea, where the country's propaganda machine turned him into a demi-god. His funeral will be held on December 28.

On the streets of the South Korean capital, Seoul, however, reaction to the death of a man whose country had threatened to turn the city into a "sea of fire" ranged from indifference to overjoyed.

"The whole earth should celebrate it as much as Christmas," said Kim Ok-tae, a 58-year old pastor. "I'm not at all afraid. I don't see any likelihood of North Korea lashing out."

REGIONAL THREAT

North Korea, which tested a nuclear device in 2006 and again in May 2009, is seen as one of the greatest threats to regional security.

Last year, the secretive North unveiled a uranium enrichment facility, giving it a second route to make an atomic bomb along with its plutonium program.

Victor Cha, a Korea expert with the Center for Strategic and International Studies think-tank in Washington, said communication between China, the United States and South Korea was vital.

"Because these are the three key players when it comes to instability in North Korea. And the Chinese have been reluctant to have any conversations on this," he said.

"Now the situation really calls for it. It will be interesting to see how much the Chinese will be willing to have some sort of discussion."