Video - Breaking News Videos from CNN.com North Korea threatened Thursday to launch a "sacred war" after South Korea completed large military exercises near the volatile inter-Korean border. "The revolutionary armed forces of the DPRK (Democratic People's Republic of Korea) are getting fully prepared to launch a sacred war of justice of Korean style based on the nuclear deterrent at anytime necessary," North Korea's defense minister Kim Yong Chun said, according to the state-run news agency. "The South Korean puppet forces perpetrated such grave military provocation as renewing their shelling against the DPRK during their recent exercises for a war of aggression in the West Sea of Korea," Chun said. "This indicates that the enemy's scenario for aggression aimed at the start of another Korean War, has reached the phase of its implementation. U.S. officials appear unimpressed by the threats. "Unfortunately, North Korea is back to its old belligerent tricks," wrote State Department spokesman P.J. Crowley in a Twitter posting. "We need constructive actions, not heated rhetoric." South Korean President Lee Myung-bak had harsh words for North Korea on Thursday. "In the case of another surprise attack, the country must launch a merciless counterattack," Lee said.