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North Korean dictator Kim Jong Il's iron grip on his isolated nation has finally loosened with news of his death -- but the future of the nation is still very much under an ominous question mark.

The flamboyant yet enigmatic ruler died of heart failure, state television has reported. He was 69.

In a special broadcast on Monday local time, state media reported that Kim died Saturday due to "great mental and physical strain." The report said Kim was on a train conducting a "high intensity field inspection."

An autopsy conducted on Sunday "fully confirmed" the diagnosis, the state media report said.

"It is the biggest loss for the party ... and it is our people and nation's biggest sadness," an anchorwoman said through tears. She said the nation must "change our sadness to strength and overcome our difficulties."

Kim's health had been a question mark for years. He is believed to have suffered a stroke in 2008, and reports suggested he suffered from both diabetes and heart disease.

In recent photos and video from trips to China and Russia, the man known for his bouffant hairstyle appeared at times vigorous, and at other times extremely frail.

News of Kim's death spurred South Korea to put its military on "high alert," according to media reports in that country. South Korean President Lee Myung-bak is also said to have convened a special meeting of his national security council.

A statement from the White House said U.S. President Barack Obama is aware of reports of Kim's death, and is in close touch with the leaders of both South Korea and Japan.

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

News of Kim's death comes as the communist country has been preparing for succession. Kim Jong Il inherited power after his father, revered North Korean founder Kim Il Sung, died in 1994.

In September 2010, Kim Jong Il named his third son, Kim Jong Un, as his successor, placing him in high-ranking posts.

CTV's Beijing Bureau Chief Ben O'Hara-Byrne, who has reported from North Korea, said the process to install Kim Jong Un as leader "was very early on in its stages," and it is unclear how smooth the transition will be.

He said North Korea will likely remain closed off for a period of mourning.

"I don't think we'll see North Korea open up any time soon," he told Â鶹´«Ã½ in a telephone interview.

He said regional neighbours China and South Korea "will have to reassess who they're dealing with now."

Kim Jong Il, once described by former U.S. president George W. Bush as a "tyrant," had been groomed for 20 years to lead the communist nation founded by his guerrilla fighter-turned-politician father.

There are few details known about the country and the man known affectionately as "Dear Leader," and there are also conflicting reports as to when the leader was born.

Although legend has it Kim was born on Mount Paekdu in 1942, Soviet records claim he was born in Siberia in 1941 while his parents were in exile.

Kim Jong Il, who graduated from Pyongyang's Kim Il Sung University, was 33 when his father tapped him as his successor.

Even before he took over as leader, there were signs the younger Kim would maintain -- and perhaps exceed -- his father's hard-line stance. During his years in power he carried out his father's "military first" policy by devoting much of the country's resources to troops. He built the world's fifth largest military even while the country suffered through a prolonged famine.

Kim also strived to build up the country's nuclear arms arsenal, culminating in North Korea's first nuclear test explosion, in October 2006. Another test followed in 2009.

His pursuit of nuclear weapons and repeated threats to South Korea and the U.S. stoked worries that fighting might break out once again on the Korean peninsula or that North Korea might provide support to terrorist movements.

Though the Korean War ended more than 50 years ago with a ceasefire, the two sides are still technically in a state of war.

"Look, Kim Jong Il is a dangerous person. He's a man who starves his people. He's got huge concentration camps. And ... there is concern about his capacity to deliver a nuclear weapon," Bush said in 2005.

Regional leaders signed an aid-for-disarmament pact with North Korea in 2007, but the delivery of that aid has been spotty at best despite the desperate need for food and other supplies.

While North Koreans suffered from a lack of food, Kim enjoyed a lavish lifestyle that included fine food and wine.

Kim's distinctive appearance was often mocked. Short and pudgy at 5-foot-3, his peculiar attire of jumpsuits and sunglasses was ridiculed in scores of films, including "Team America: World Police."

Kim is believed to have had a wide range of interests, from Western cinema to cigars, cognac and fine wine.

He reportedly produced several films over the years, and a South Korean film director claims he and his wife were kidnapped by Kim's regime in the 1970s and held for a decade, forced to make propaganda films until they were able to escape during a trip to Austria.

There have been only a handful of insider accounts about Kim.

One came from Madeline Albright, former U.S. Secretary of State under former president Bill Clinton. She met Kim during visits to Pyongyang and described the dictator as intelligent and well-informed.

Another fascinating account came from Konstantin Pulikovsky, a former Russian presidential envoy who travelled with Kim on a luxury train trip from Pyongyang to Moscow.

He wrote a book, "The Orient Express," about the journey, describing a lavish train trip complete with French wine and fresh live lobsters.

Kim's marital status was unclear but he is believed to have married once and had at least three other companions. He had at least three sons with two women, as well as a daughter by a third.

It was long believed that Kim's eldest son, 40-year-old Kim Jong Nam, was his chosen successor. However, he appeared to have fallen out of favour after being caught trying to enter Japan on a fake passport in 2001, apparently in an attempt to visit the Tokyo Disney Resort.