South Korea's military is on high alert following the death of North Korean leader Kim Jong Il.
South Korean president Lee Myung-bak convened a national security council meeting and banning all leave and travel for government officials, The Daily Telegraph reported.
Lee also ordered its military on emergency alert, increased border air patrols and asked its United States ally, which posts 28,500 troops in the south, to step up surveillance via planes and satellites.
North and South Korea ended three years of armed conflict with a cease-fire in 1953, but are technically at war as no peace treaty was signed.
Recent flare-ups have killed 50 South Koreans in recent years, The Telegraph reported.
Last November, four people were killed a South Korean island was shelled, which earlier last year a South Korean warship was torpedoed killing 46 sailors, an attack denied by Kim’s regime.
According to The Associated Press, President Barack Obama and Lee "agreed by phone to closely monitor the events in the North and cooperate."
The North's official Korean Central News Agency, meantime, urged the country to "faithfully revere respectable comrade Kim Jong Un," reference to Kim's youngest son and heir apparent.
(GlobalPost reports: Kim Jong Il dead: What's next for North Korea?)
According to the AP, North Korea's military has "pledged to uphold the leadership of comrade Kim Jong Un."
"At the leadership of comrade Kim Jong Un, we have to change sadness to strength and courage and overcome today's difficulties," North Korea said.