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

A tearful television announcer dressed in black 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.