SEOUL, South Korea — South Korean authorities were stepping up the search for sailors still missing hours after a naval ship sank near the disputed sea border with North Korea. Navy and coast guard vessels as well as air force planes were searching the waters near South Korea's Baeknyeong Island where the 1,200-ton "Cheonan" sank earlier Saturday during a routine patrolling mission. The Joint Chiefs of Staff said rescuers have picked up 58 sailors but 46 others are missing. Presidential spokeswoman Kim Eun-hye said President Lee Myung-bak ordered officials to quickly find the exact cause of the accident by keeping in mind all possibilities. The president also reconvened a security meeting and instructed officials to make utmost rescue efforts.