South Korea and the United States have agreed to carry out a massive anti-submarine training drill in the West Sea sometime next month, mobilizing a U.S. nuclear-powered aircraft carrier, in a strong protest against North Korea's sinking of the Navy corvette Cheonan. A senior government official on Thursday said the two allies "decided to hold a large-scale joint drill in June, much earlier than originally planned, to express our strong determination to take steps against North Korea."