North Korea has replaced its defense chief with a hardline general believed to have masterminded deadly 2010 attacks on South Korea.

South Korean officials said Thursday that Kim Kyok Sik was named to the post of Armed Forces Minister. He is is believed to have orchestrated the sinking of a South Korean warship and an attack on a border island that threatened to plunge the countries into all-out war.

It is the second new defense minister named by North Korea's young leader Kim Jong Un, who took over in December following his father's death. The outgoing defense minister, Vice Marshal Kim Jong Gak, lasted just seven months in office.

Seoul officials say the move is a latest in a series of shakeups aimed at tightening Kim Jong Un's grip on the 1.2 million-member military, one of the world's largest.

In July, Mr. Kim unexpectedly replaced one of the country's highest ranking officials, military chief Ri Yong Ho, with a little known general.

Pyongyang said Ri's dismissal was due to an undisclosed “sickness,” but many observers said it was part of a purge meant to dismiss leaders who were not allied closely to Mr. Kim.