A wind-fed mountain fire that broke out in North Korea was spreading rapidly into the South Korean sector of the inter-Korean border Saturday but no one was hurt, military authorities said.The blaze, first spotted in the northern side of the 4-kilometer-wide Demilitarized Zone (DMZ) at around at 4:40 a.m., quickly spread past the border and reached within 600 meters of the nearest South Korean guard posts in the area, they said.