US and South Korean warships on Monday staged anti-submarine drills as part of a major naval exercise. The two allies assembled about 20 ships including the 97,000-ton carrier USS George Washington, 200 aircraft and 8,000 personnel. Four F-22 Raptor stealth fighters were flying in and around Korea for the first time to show Washington's strong commitment to deter and defeat any provocative acts. Seoul and Washington say the four-day exercise which began on Sunday is designed to stress that future attacks will meet a decisive response. A spokesman for the South's Joint Chiefs of Staff told reporters today's maneuvers focussed on better detecting intrusions by an enemy's submarines and attacking them. The exercise is being held in international waters in the Sea of Japan, 200 kilometers south of North Korean waters.