North Korea's official news agency reported Monday that Supreme Leader Kim Jong Il has died at the age of 69.

The Korean Central News Agency said the reclusive leader died Saturday of a heart attack while on a train for one of his “field guidance” tours. The agency attributed his death to “physical and mental over-work.”

KCNA said his funeral will be held on December 28 in Pyongyang. A period of national mourning was declared from December 17 to 29.

Kim Jong Il came to power after his father, North Korea's founder Kim Il Sung, died in 1994. Reliable biographical information about Mr. Kim is scarce. He rarely appeared in public and his voice was seldom broadcast.

He may be best remembered for defying the international community and boosting North Korea's nuclear program, while millions of his countrymen and women were starving.

Late last year, Mr. Kim promoted his youngest son Kim Jong Un to the rank of four-star general, in what was seen as a bid to extend the world's only communist family dynasty to a third generation.

The KCNA on Monday urged people to follow Kim Jong Un, who is believed to be around 28 years old.