South Korea's military is on high alert after a North Korean patrol boat was damaged yesterday during a clash between the two Korea's navies. A spokesman for South Korea's Joint Chiefs Of Staff said the South's army, navy and air force are closely monitoring the activities of the North's military. But South Koran officials say so far they have not noted any unusual troop movement by the North. They have sent two additional warships to guard the maritime border in the Yellow Sea. A top presidential aide responsible for South Korean security affairs says the North may take its time and wait for the best opportunity to retaliate.