Seoul put South Korean forces on high alert and Pyongyang urged an increase in its "military capability" as the death of North Korea's enigmatic leader Kim Jong Il spurred fresh security concerns in the tense region.

The 69-year-old "dear leader" of the reclusive communist state died of a heart attack on Saturday, state news outlets reported Monday. The ruling Worker's Party proclaimed his youngest son, Kim Jong Un, "the great successor," indicating he would assume his father's post.

In the South, across one of the world's most heavily fortified borders, President Lee Myung-bak canceled the rest of his Monday schedule and put all members of South Korea's military on "emergency alert," his office said. The two nations never signed a peace treaty following the Korean War of the early 1950s, leaving the two nations technically at war.

After an emergency Cabinet meeting Monday, Lee asked South Koreans "to go about their lives."

"For the sake of the future of the Republic of Korea, peace and stability on the Korean Peninsula is more important than anything else. It should not be threatened by what has happened," he said.

And Kim Young-mok, the South Korean consul in New York, told CNN's "American Morning" that Seoul's top priority is to avoid "anything troublesome."

"I don't think that North Korea can afford some provocation at this point, but we must make sure that everything is OK," he said.


North Korea: Our leader is dead

Can the son of the 'Dear Leader' lead? A look back at the life of Kim Jong Il

North Korea tested nuclear weapons in 2006 and 2009. Monday, it fired a short-range missile over the East Sea -- but Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told CNN that U.S. officials have seen no sign of any movement of North Korean forces across the Demilitarized Zone that separates the North and South.

North Korean officers had reported plans for its missile test to the commission that monitors the 1953 armistice, Dempsey said during a trip to Germany, and he said no heightened alert has been issued for the nearly 30,000 U.S. troops in South Korea.

South Korea also raised its alert level for possible cyberattacks one notch, to the third of four tiers, the country's leading communications office reported. North Korea launched two series of attacks on South Korean government websites and civilian web pages, including those of banks, portals and media organizations, in 2009 and 2011.

In Washington, the White House said President Barack Obama spoke with Lee on Monday morning. The two leaders agreed to stay in "close touch as the situation develops," it said.

In the North, a tearful state television broadcaster reported Kim's death early Monday, and the network showed residents weeping uncontrollably on the streets of Pyongyang following the news.

"My leader, what will we do? It's too much! It's too much!" one person sobbed. "Leader, please come back. ... You cannot leave us. We will always wait for you, leader."

The network said Kim died of "overwork" while "dedicating his life to the people." And the official news agency KCNA said Kim suffered "great mental and physical strain" while on a train.

There was no independent confirmation of the circumstances reported by the government, which routinely touted the seemingly mythic achievements of the "dear leader" during his lifetime. But he had appeared frail in his last public appearances, and speculation about his health had persisted for some time.

KCNA acknowledged that Kim had been treated for "cardiac and cerebrovascular diseases for a long period." He suffered a heart attack on Saturday and couldn't be saved despite the use of "every possible first-aid measure," according to the agency.

This Just In: Up-to-the minute news on the death of Kim Jong Il

The deceased leader's body will remain for a week at the Kumsusan Memorial Palace in Pyongyang -- where the body of his father, North Korea's founder Kim Il Sung, also lies. Memorial services will follow on December 28 and 29.

"We should increase the country's military capability in every way to reliably safeguard the Korean socialist system and the gains of revolution," the National Funeral Committee said.

Bill Richardson, a former U.S. ambassador to the United Nations who has visited North Korea eight times, said his initial reaction to Kim's death was "extreme concern." He said he is more concerned about stability in the region now than before news broke of Kim's death.

"North Korea, the peninsula is a tinderbox," said Richardson, who has brokered diplomatic deals in the North. "The issue is, will there be stability in the North Korean leadership? Will they continue their recent efforts of engaging South Korea and the United States over food aid, over nuclear talks?"

Richardson said South Korea was right to go on alert -- "but I think now's the time to just lie low, watch things as they develop."

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Kim took power in 1994, when his father died of a heart attack at age 82. The reclusive leader was a frequent thorn in the side of neighboring South Korea, as well as the United States.

Under his leadership, North Korea was largely closed off to outside influences, fearful of threats from its neighbors. At the same time, it also sought international aid after extensive famines contributed to the deaths of tens of thousands of people.

Both North Korea and South Korea have shown signs of concession in recent years. But relations between the two rival nations soured yet again when the South accused the North of launching an attack on the South Korean island of Yeonpyeong, killing two South Korean marines and two civilians.

And South Korea says a North Korean torpedo attack was to blame for the 2010 sinking of the South Korean warship Cheonan, which left 46 Southern sailors dead. The North has denied the accusation.

Now the question is what path his untested son, believed to be in his late 20s, will take the Stalinist "Hermit Kingdom."

Kim Young-mok said the North has built up its military and pursued a nuclear bomb "while children are starving."

"This causes a lot of human disasters," he said. "So what worries me is not this Kim Jong Un's personality, but the priority of the North Korean leadership."

The ruling Workers' Party confirmed called the younger Kim, who was named a four-star general in 2010, "the "great successor to the revolutionary cause" and the "outstanding leader of our party, army and people."

"Kim Jong Un's leadership provides a sure guarantee for creditably carrying to completion the revolutionary cause of Juche through generations," the party announced in a letter posted on KCNA.

The philosophy of "juche," or self-reliance, is the basis of North Korea's reclusive nature.

Mike Chinoy, a senior fellow at the U.S.-China Institute, said he expects North Koreans to "rally around the flag (and) hunker down" in the short term. But given the nation's deep-rooted economic and other problems, maintaining that unity and control without a overarching figure like Kim Jong Il in place may be more difficult.

"The deeper questions come over the long term," Chinoy said.