North Korea's military has threatened to strike back if South Korea goes ahead with a planned live-fire drill on a border island the communist state shelled last month. According to reports, South Korea is set to hold the one-day drill some time between Saturday and Tuesday. In a statement, North Korea's military said the second and third self-defensive blows that cannot be predicted will be dealt if the drill goes ahead . It said the intensity and range of the firepower will be more serious than on November 23. The bombardment of Yeonpyeong island last month killed two marines and two civilians, injured 18 people and damaged dozens of homes.