An emotional North Korean newsreader was heard fighting back tears on Monday as she announced the death of the country's mercurial and enigmatic long-time leader, Kim Jong Il.

In a "special broadcast" 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 December 17 during a "high intensity field inspection."

He was 69.

It said an autopsy was done on December 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 apparent successor by placing him in high-ranking posts.