North Korean leader Kim Jong-un visited front-line military units responsible for the shelling of a South Korean island in 2010, the North's state media reported Sunday, ahead of a planned joint military exercise by the South and the U.S.

The Korean Central News Agency (KCNA), monitored in Seoul, said Kim inspected units under the North Korean army's 4th Corps in the southwestern region. The report didn't specify when Kim made the trip.

According to the KCNA, Kim inspected the 1st and 4th Battalions. The 4th Battalion is the unit that launched artillery shots on South Korea's Yeonpyeong Island in November 2010.

The report also said Kim called on the service members to "make a powerful retaliatory strike" at South Korea, "should the enemy intrude even 0.001mm into the waters of the country where its sovereignty is exercised."

It added that Kim called the southwestern sector "a hot spot where a war may break out any moment due to the enemy's reckless provocations for aggression."

The announcement of Kim's trip to these units comes a day before South Korea and the U.S. are scheduled to kick off their joint annual Key Resolve military exercise. It will mobilize more than 200,000 troops from the allies to help strengthen their readiness against potential North Korean provocations.

North Korea has routinely denounced the Key Resolve drills as precursors for an invasion, while Seoul and Washington have countered that they are defensive in nature.

On Saturday, Pyongyang vowed to launch a "sacred war" against the South and the U.S., saying a war has been declared against North Korea and it will fight back to protect its own security. (Yonhap News)