Kim's comments on bolstering the military strength on the border to make it an "impregnable fortress" came at a meeting on Sunday of commanders of divisions and brigades across the army, KCNA said on Monday.
He called for adjusting the training system and expanding practical drills to reflect changes in modern warfare and redefine operational concepts in North Korea's military, according to KCNA.
Kim stressed the need for vigilance against the "arch enemy," a term North Korea has used for South Korea.
The two Koreas are still technically at war after their 1950-53 conflict ended with an armistice rather than a peace agreement.
South Korea's Unification Ministry on Monday said it appeared to be Kim's first reported meeting with division and brigade commanders since he took power, adding that Seoul would continue to manage military tensions and seek to build trust.
South Korea's Joint Chiefs of Staff on Monday said North Korean troops had stepped up fortification work since March in areas near the land border between the two Koreas, including building walls.
Hong Min, a senior research fellow at the Korea Institute for National Unification in Seoul, said Kim's reference to fortifying the "southern border" suggested Pyongyang may also strengthen its military presence on its maritime boundaries with the South, such as the disputed Northern Limit Line (NLL).
Kim's references to modern warfare and redefining operations "in all spheres" likely reflected lessons Pyongyang has drawn from the war in Ukraine and Middle East conflicts, including the use of drones, precision strikes and electronic warfare, Hong said.
