South Korea's navy fired warning shots today to drive back a North Korean boat which was fishing illegally in southern waters amid high tension along the Yellow Sea border. The Joint Chiefs of Staff said in a statement that the boat retreated to its side about two hours and 40 minutes after it had intruded across the disputed border. The intrusion came at a sensitive time, with the South's military and police on high alert before next week's Group of 20 summits in Seoul. In March a South Korean corvette sank in the area with the loss of 46 lives. Cross-border tensions have been high since Seoul, citing a multinational investigation, accused Pyongyang of torpedoing the ship. It denies the charge.