Kim, who took over the reins of the isolated Communist country after the December death of his father, Kim Jong Il, issued his orders as the military's supreme commander ahead of the joint U.S.-South Korea military drills in the region, which began Monday.
A report by North Korea's Korean Central News Agency said Kim visited the front-line military units in the southwest region where he inspected combat positions of the 4th battalion to learn in detail about its combat preparations.
The report recalled the battalion's November 2010 deadly shelling of the South Korean island in retaliation "against South Korean puppet warmongers' reckless shelling into territorial waters" of North Korea.
Kim underlined the need to "further increase the density of firing in the future" and said the southwestern sector of the front is a hot spot "where a war may break out any moment due to the enemy's reckless provocations for aggression."
"He ordered them to make a powerful retaliatory strike at the enemy, should the enemy intrude even 0.001 mm into the waters of the country where its sovereignty is exercised," KCNA said.
North Korea Monday condemned the U.S.-South Korea military, warning its army and people are ready for a war with the two, KCNA reported.
Pyongyang calls the drills "an unpardonable infringement upon the sovereignty and dignity" of North Korea.