Kim Jong Un took over as leader of North Korea following the December death of his father, Kim Jong Il.
An official from the South Korean Unification Ministry told the region's Yonhap news agency on condition of anonymity that the new leader was conducting a "loyalty check."
"After he officially took over power last April, Kim Jong Un has been conducting a task to verify loyalty or (existence of) corruption of key figures," the official said.
In July, the North Korean leader was awarded the title of marshal, making him the supreme commander of the country's military.
The announcement followed shake-ups in Kim's inner circle of advisers. Ri Yong Ho was removed from his post as army chief for what the government said was "illness." Gen. Hyon Yong Chol was promoted to the rank of vice marshal following Ri's ouster. Yonhap's source said Hyon's rank was recently downgraded, however.
Yonhap reports Kim's action may be in response to the October defection of a North Korean soldier or a sign the leader is trying to shift power away from the military.