Seoul, South Korea — North Korea's military said Monday its recent barrage of missile tests were practices to "mercilessly" strike key South Korean and U.S. targets such as air bases and operation command systems with a variety of missiles that likely included nuclear-capable weapons. The North's announcement underscored leader Kim Jong Un's determination not to back down in the face of his rivals' push to expand their military exercises. But some experts say Kim also used their drills as an excuse to modernize his nuclear arsenal and increase his leverage in future dealings with Washington and Seoul. North Korea fired dozens of missiles and flew warplanes toward the sea last week - triggering evacuation alerts in some South Korean and Japanese areas - in protest of massive U.S.-South Korean air force drills that the North views as an invasion rehearsal. U.S. and South Korean officials responded by saying they would further enhance their joint training events and warned the North that