The Korean Central News Agency reported Kim visited the artillery unit on the island of Mudo, South Korea's Yonhap News Agency said. The island is in the area where joint U.S.-South Korean naval exercises are to be held from Monday until Aug. 31.
The unit Kim visited shelled Yeonpyeong Island, which is claimed by both countries, in November 2010. Two civilians and two South Korean Marines were killed and many buildings were damaged.
"He solemnly declared that if the enemy dares recklessly preempt firing and even a single shell drops on the territory of the DPRK, the KPA should lead the battle to a sacred war for national reunification, not confining it to a local war on the southwest region," the KCNA said in an English-language story. "He also ordered it to turn the west sea into a graveyard of the invaders."
Kim became head of the North Korean state, which uses the name Democratic Republic of North Korea, when his father died in December 2011. He is the grandson of Kim Il Sung.