North Korea\'s new leader has visited an army unit that shelled a Southern island in 2010, state media said Sunday as Pyongyang threatens \"sacred war\" over a US-South Korean military exercise. Kim Jong-Un, who took power in the North after his father died in December, met the 1st and 4th Battalions of the Korean People\'s Army 4th Corps at a base near the border with the South, Pyongyang\'s official KCNA news agency said. It added that the 4th Battalion carried out the shelling of Yeonpyeong island in November 2010, which left four South Koreans including two civilians dead. Kim was quoted saying the area was a \"hot spot where a war may break out any moment due to the enemy\'s reckless provocations for aggression\", urging troops to stay alert over \"the enemy\'s preparations for a new war of 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. It did not specify when the visit took place, but the report came only a day before the start of Key Resolve, a joint US-South Korean computerised command post exercise, which will continue until March 9. Separately, the joint air, ground and naval field training exercise Foal Eagle will be held from March 1 to April 30. The North\'s National Defence Commission (NDC) has denounced the annual joint exercises as a \"silent declaration of war,\" describing them as \"unpardonable war hysteria.\" \"Our army and people will foil the moves of the group of traitors to the nation and warmongers at home and abroad for a new war with a sacred war of our own style,\" the NDC said on Saturday. Cross-border tension has been high since Seoul accused Pyongyang of torpedoing a warship at the loss of 46 lives near the tense sea border off the west coast in March 2010. The North denied involvement but went on to shell Yeonpyeong, sparking brief fears of war. The communist state\'s rhetoric has taken an increasingly hostile tone since Jong-Un, believed to be in his late 20s, took over from his father and longtime leader Kim Jong-Il, who died of a heart attack in December. The North last week vowed \"merciless retaliatory strikes\" if any shells landed in waters claimed by Pyongyang during a live-fire artillery exercise by the South near the disputed border on the Yellow Sea. But in the event it took no military action in response to the drill.