Seoul, Dec 19 (IANS): South Korea was in a state of high alert Monday following the death of North Korea's top leader Kim Jong-il.

Soon after the North Korea's official KCNA news agency reported Kim's death, South Korean President Lee Myung-bak convened the National Security Council (NSC) to discuss follow-up measures with cabinet ministers.

North Korean leader Kim Jong-il died Saturday during a train journey "from a great mental and physical strain". He was 69, the country's official KCNA news agency reported Monday.

Lee reportedly ordered all government employees be put on emergency alert.

South Korea's Joint Chiefs of Staff (JCS) put the military on alert and called an emergency meeting, reported Xinhua.

Authorities have stepped up border surveillance, but no unusual activity has been detected yet, according to Seoul-based Yonhap News Agency.

JCS officials are reportedly considering raising both defense readiness and surveillance status.

South Korean police also intensified vigilance.

KCNA reported that Kim died "from a great mental and physical strain at 08.30 Dec 17, 2011, on train during a field guidance tour".

Kim, who had received medical treatment for his cardiac diseases for a long period, suffered "an advanced acute myocardial infarction, complicated with a serious heart shock", on the train.

The report said that every possible first-aid measure was taken immediately, which failed to save Kim's life, adding that an autopsy Sunday "fully confirmed the diagnosis of his diseases".