Story highlights Kim Jong Un executed about 140 senior North Korean officials to solidify his power

Experts say the dictator is ruthless and keen to show he is a "great leader"

(CNN) A new report claims North Korean dictator Kim Jong Un has ordered 340 people to be executed since he came to power in 2011.

The Institute for National Security Strategy, a South Korean think tank, released "The misgoverning of Kim Jong Un's five years in power" on Thursday, detailing how the North Korean leader uses executions to tighten his hold on power.

Of those killed, about 140 were senior officers in the country's government, military and ruling Korean Worker's Party.

North Korean leader Kim Jong Un (C) during a combat drill in an undated photo released by state media.

RAND Corporation senior defense analyst Bruce Bennett told CNN Kim had shown an "extreme" level of brutality and ruthlessness since 2011.

"For example, in the five years he has served as leader of North Korea, he has purged (his) Defense Minister five times, while his father changed his Defense Minister only three times in his 17 years ... and two of those changes were because (they) died of old age," he said.