North Korea threatened on Friday to attack South Korea if activists proceeded with a plan to send leaflets across the border criticizing the North’s government. South Korea’s military said it would immediately strike back if the North did so. An umbrella group of anti-North Korean groups, led mostly by defectors from the North, says it plans to release balloons carrying propaganda leaflets on Monday in Imjingak, near the border with North Korea. Activists have conducted similar balloon launchings before. The North’s official Korean Central News Agency said, “The leaflets are a most undisguised act of psychological warfare, a violation of the armistice and an intolerable act of war.” It urged South Koreans to leave the targeted area. South Korea’s defense minister noted that North Korea had issued similar threats before about leaflets from the South without acting on them.