The UN Secretary-General Ban Ki-moon today urged Central Asian leaders to step up efforts to solve the problem of drying up of Aral Sea. Once the world's fourth-largest lake, the sea has shrunk by 90 per cent. The UN Secretary General toured the sea by helicopter as part of a visit to the five countries of former Soviet Central Asia.The Aral Sea catastrophe is one of Ban's top concerns on his six-day trip. He urged the countries' leaders to set aside rivalries to cooperate on repairing some of the damage.