South Korea vowed to sternly retaliate against any North Korean provocations on Thursday, a day after Pyongyang warned of an "unimaginable holocaust" in response to South Korea's upcoming joint military exercises with the United States. South Korea also urged North Korea not to harm inter-Korean relations, noting that North Korean leader Kim Jong-un called for an end to "slander and calumny" in his New Year's address. Defense ministry spokesman Kim Min-seok said the North's latest threat that the exercises may push inter-Korean ties toward catastrophe is "self-contradictory."    South Korea and U.S. forces plan to conduct command post exercise Key Resolve and joint field training exercise Foal Eagle between late February and April to enhance joint combat readiness and deter threats from the North. Currently, about 28,500 American troops are stationed in South Korea, a legacy of the 1950-53 Korean War. Despite repeated assurances by Seoul and Washington that the drills are defensive in nature, the North suspects the military exercises could be a rehearsal for a nuclear war against it. "We will sternly retaliate against North Korea if the North carries out military provocations," Kim said in a regular briefing. South Korea has said the North may stage provocations against South Korea as it seeks to forge internal unity following the last month's execution of its leader Kim's once-powerful uncle, Jang Song-thaek. An official of the Unification Ministry also pressed the North to stop "its behavior that undermines confidence," and show a responsible attitude if the North truly wants to improve ties. The comments by South Korean officials came a day after the North condemned the planned military drills by South Korea and the U.S. as a "serious military provocation" and an outright challenge to its good faith. "North-South ties will plunge into a deadlock and unimaginable holocaust and disaster will follow should they go ahead with the nuclear war drills and make military provocations," the North's Committee for the Peaceful Reunification of the Fatherland said late Wednesday.