North Korean leader Kim Jong-il died of a heart attack while on a train trip, state media said on Monday, December 19, sparking immediate concern over who is in control of the reclusive state and its nuclear program, Reuters reports

A television announcer said the 69-year old had died on Saturday of physical and mental over-work on his way to give "field guidance."

Kim Jong-un, Kim Jong-il's youngest son, is seen as the leader-in-waiting after he was appointed to senior political and military posts in 2010.

North Korea's official KCNA news agency said the elder Kim died at 8:30 a.m. on Saturday (6:30 p.m. EST on Friday) after "an advanced acute myocardial infarction, complicated with a serious heart shock." Kim had suffered a stroke in 2008, but had appeared to have recovered from that ailment.

South Korea, still technically at war with the North, placed its troops and all government workers on emergency alert, Yonhap news agency reported. But Seoul's Defense Ministry said there were no signs of any unusual North Korean troop movements.