The leader of North Korea, Kim Jong-il, died at the age of 69.

North Korea’s state-run television made the announcement Monday, saying Kim died of physical and mental over-work, according to BBC. He died on a train while visiting an area outside North Korea’s capital, Pyongyang, the report said.

In 2008 Kim suffered a stroke, and according to the Associated Press, was believed to have diabetes and heart disease.

Kim has been in power since 1994, taking over as the clear successor after the death of his father, Kim Il-sung. In the past few years, Kim has begun transferring power to his own youngest son, who was appointed several high-ranking positions, including being named vice chairman of the nation’s National Defense Commission in 2011 – North Korea’s most powerful military and political body.

Following the announcement, South Korean military has been put on alert, and the country’s National Security Council is convening for an emergency meeting.