The night a torpedo-armed North Korean submarine allegedly sank a South Korean patrol ship, the U.S. and South Korea were engaged in joint anti-submarine warfare exercises just 75 miles (120 kilometers) away, military officials told The Associated Press. The blast that sank the Cheonan, the worst South Korean military disaster since the 1950-53 Korean War, showed how impoverished nations such as North Korea can still inflict heavy casualties on far better equipped and trained forces, even those backed by the might of the U.S. military. Defense Secretary Robert Gates said Friday that plans for more joint U.S.-South Korea anti-submarine exercises, announced after the sinking of the Cheonan, are on hold awaiting United Nations action on the incident. On Friday, South Korea referred North Korea to the U.N. Security Council over the sinking. In part, Gates said, there is concern about instigating another rash act by the North Koreans. Two months after the sinking, U.S. officials for the first time disclosed details of the joint naval exercise held the same day as the attack on the Cheonan. Forty-six South Korean sailors died on the warship, which was not involved in the exercise but on routine patrol near disputed waters. -- SPA