Among those killed were four senior members of Pyongyang's famous Unhasu Orchestra, where Kim's wife, Ri Sol-ju, had been a singer before she married him in June 2012.
They were shot by firing squad on spying charges, South Korean intelligence officials said.
In 2013, Kim purged and executed his uncle, General Jang Song-thaek, once considered the second most powerful man in Pyongyang's leadership circle, after he was branded a traitor and accused of a string of alleged crimes.
Kim has repeatedly reshuffled close aides and senior officials since taking office.
By Ruchi Singh