Video After The Jump
(CNN) -- 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.
A tearful state TV broadcaster reported Kim's death Monday. She said the 69-year-old leader died Saturday due to "overwork" while "dedicating his life to the people."
North Korea's official KCNA news agency said Kim suffered "great mental and physical strain" while on a train. Kim, who had been treated for "cardiac and cerebrovascular diseases for a long period," suffered a heart attack on Saturday and couldn't be saved despite the use of "every possible first-aid measure," according to the agency.
In the country where Kim was revered as "dear leader," passers-by wept uncontrollably on the streets of Pyongyang.
"My leader, what will we do? It's too much! It's too much!" one person sobbed on state television. "Leader, please come back. ... You cannot leave us. We will always wait for you, leader."
Kim's youngest son, Kim Jong Un, will likely take over the reins. A letter from the ruling Workers' Party on Monday dubbed him "the great successor."
The deceased leader's body will remain for a week at the Kumsusan Memorial Palace in Pyongyang -- where the body of his father, Kim Il Sung, 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
For its part, South Korea's National Police Agency ordered officers across the country to be ready for overtime shifts. President Lee Myung-bak canceled the rest of his Monday schedule, and all members of South Korea's military were placed on "emergency alert," his office said.
Under the alert, South Korean forces will closely monitor North Korean troop movements and tighten security measures at sea, according to the ministry of defense.
Following the Korean War in 1950, the two nations never formally signed a peace treaty and remain technically at war -- separated by a tense demilitarized zone.
"South Korea's concern is warranted, frankly, because an insecure North Korea could well be an even more dangerous North Korea," a U.S. official said.
But the demilitarized zone between the Koreas remained peaceful on Monday.
"We have not seen any unusual movement ... from North Korea," said Choi Jong Kun, a professor at Yonsei University in Seoul. "We will have to watch and see."
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.
U.S. President Barack Obama spoke with Lee on Monday morning, and the two agreed to stay in "close touch as the situation develops," the White House 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 abut 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?"
However, Richardson said, "I think it's important that if the signs are positive that there's a stable succession -- and we don't know that -- that we engage North Korea, that we proceed with the humanitarian aid. People are starving there. Keep it non-political, but at the same time, keep a very watchful eye, standing beside our ally, South Korea."
Kim Jong Il had been in power since 1994 when his father -- the nation's founder -- died of a heart attack at age 82. The obscure leader was a frequent thorn in the side of neighboring South Korea, as well as the United States.
North Korea's nuclear program and international attempts to hinder its nuclear weaponry potential put Kim at odds with many world leaders in recent years, as did his governing style.
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. Pyongyang has expressed willingness to engage with countries involved in multilateral talks aimed at North Korea's denuclearization, while Seoul recently sent humanitarian aid through U.N. agencies to help the malnourished population in the North.
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. Two South Korean marines and two civilians were killed.
Reports surfaced in recent years about Kim's failing health. But North Korean news reports earlier this fall indicated that Kim had been traveling around the country and visiting China, a big change from 2009 when he was thought to be ill with cancer.
On Monday, the ruling Workers' Party confirmed that Kim will be succeeded by his youngest son, whom the party called 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.
The son started his military career as a four-star general and in recent years was given more official duties by his father.
"This has been in place for a while," said Mike Chinoy, a senior fellow at the U.S.-China Institute.
Chinoy said he expected that, in the short-term, North Koreans would "rally around the flag (and) hunker down." 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.
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