A major South Korean naval exercise went into its third day today, as North Korea issued a fresh warning against what it termed a prelude to a war of aggression. South Korean defence ministry said the drill far south of the disputed maritime border in the Yellow Sea, was focused on defence against North Korea's guided missiles and submarines. It said the exercise is one of a series planned in coming months in a show of force against the North.Yonhap news agency quoting a military official said some 20 navy ships carried out anti-missile training in the Yellow Sea. The drill included anti-aircraft and anti-submarine battles in the Yellow Sea as batteries stationed in an island near the sea border continued live-fire training.North Korea said the naval exercise in the tense Yellow Sea was aimed at triggering an armed conflict to start a war of aggression. Its communist party's official daily, Rodong Sinmun, said the North's military has threatened the most powerful retaliation if the South triggers a conflict during the exercise.