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South Korean armed forces have engaged in drills against what it called a possible maritime intrusion by the Democratic People's Republic of Korea.
Friday's the second day of the five-day anti-submarine exercises in waters where its warship sank in March.
Officials say the large-scale naval drills, which involve 4,500 troops from the country's army, navy and air forces, some 20 vessels and 50 fighter jets, are aimed at deterring further aggression from the DPRK but are "defensive" in nature.
Seoul began the exercise off the west coast of the divided Korean Peninsula on Thursday, where one of its navy corvettes sank due to a presumed DPRK torpedo attack.
The DPRK has repeatedly denied its role in the sinking of the ship, and warned recently of a "powerful physical retaliation."