In a video aired by KRT, Kim was seen smiling and clapping as shirtless soldiers used their hands to smash bricks placed on colleagues' chests and others broke chains wrapped around their bodies in shows of strength.
During the exhibition, Kim said his country's weapons development is necessary in the face of hostile policies from the United States and a military buildup in South Korea, according to state media.
In a speech, Kim said he was building an "invincible" military to counter US hostility.
"The US has frequently signalled it’s not hostile to our state, but there is no action-based evidence to make us believe that they are not hostile,” Kim said Monday.
"The US is continuing to create tensions in the region with its wrong judgments and actions."