Kim's comments came during a visit to a military unit near the port of Nambo on the anniversary of the day his late father was named supreme commander of the North Korean army, the South Korean news agency Yonhap reported Wednesday.
"He instructed the unit to put utmost spurs on rounding off its combat readiness with the firm viewpoint ... always bearing in mind that a war breaks out without any prior notice," the North's Korean Central News Agency said in a statement.
Kim's visit coincided with a visit by South Korean President Park Geun-hye to a front-line unit. She ordered soldiers to deal "sternly and mercilessly" with any North Korean provocations.
It was Park's first visit to the front line since being elected in February.