North and South Korea have briefly exchanged naval fire along their disputed western sea border. Military officials from the south say they suffered no casualties, but the North Korean ship sustained heavy damage before retreating. They say the incident began when the North's patrol boat crossed the western sea border on Tuesday morning, drawing warning shots from one of their navy vessels. The North Korean boat then opened fire and the South's ship returned fire before the North's vessel sailed back toward its waters. There was no immediate comment from North Korea on the clash. Navies of the two countries have fought deadly skirmishes along the border, which they have yet to agree on more than 50 years after the end of the Korean War. Instead, they rely on a line that the then-commander of UN forces, which fought for the South, drew unilaterally at the end of the conflict. North Korea last month accused South Korean warships of broaching its territory in waters off the west coast and warned of a clash in the zone, which is a rich fishing area.