North Korean leader Kim Jong Un's uncle's execution has sparked fears of instability in the secretive nuclear-armed state.
South Korean Unification Minister Ryoo Kihl-jae warned that the purge could be followed by military moves from Pyongyang, including another nuclear test, the BBC reports.
South Korean defence chief Kim Kawn-jin also promised "heightened readiness", saying that the execution could be seen as part of a "reign of terror" by the North Korean leader.
A tribunal had examined Jang Song Thaek's crimes, which included attempting to overthrow the state by all sorts of conspiracies and disgraceful methods with an ambition to grab the supreme power of the party and the state, and he was executed by federal safeguards following the court orders.