PYONGYANG, North Korea — Kim Jong Il, North Korea's mercurial and enigmatic longtime leader, has died of heart failure. He was 69.

In a "special broadcast" Monday from the North Korean capital, state media said Kim died of a heart ailment on a train due to a "great mental and physical strain" on Dec. 17 during a "high intensity field inspection." It said an autopsy was done on Dec. 18 and "fully confirmed" the diagnosis.

Kim is believed to have suffered a stroke in 2008, but he had appeared relatively vigorous in photos and video from recent trips to China and Russia and in numerous trips around the country carefully documented by state media. The communist country's "Dear Leader" — reputed to have had a taste for cigars, cognac and gourmet cuisine — was believed to have had diabetes and heart disease.

"It is the biggest loss for the party ... and it is our people and nation's biggest sadness," an anchorwoman clad in black Korean traditional dress said in a voice choked with tears. She said the nation must "change our sadness to strength and overcome our difficulties."

South Korean media, including Yonhap news agency, said South Korea put its military on "high alert" and President Lee Myung-bak convened a national security council meeting after the news of Kim's death. Officials couldn't immediately confirm the reports.

The news came as North Korea prepared for a hereditary 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 unveiled his third son, the twenty-something Kim Jong Un, as his successor, putting him in high-ranking posts.

Traffic in the North Korean capital was moving as usual Monday, but people in the streets were in tears as they learned the news of Kim's death. A foreigner contacted at Pyongyang's Koryo Hotel said hotel staff were in tears.

Asian stock markets moved lower amid the news, which raises the possibility of increased instability on the divided Korean peninsula.

South Korea's Kospi index was down 3.9 percent at 1,767.89 and Japan's Nikkei 225 index fell 0.8 percent to 8,331.00. Hong Kong's Hang Seng slipped 2 percent to 17,929.66 and the Shanghai Composite Index dropped 2 percent to 2,178.75.