UN chief Ban Ki-moon on Wednesday appealed to world leaders to make a \"personal priority\" of breaking the negotiating deadlock ahead of a major UN sustainable development summit this month. More than 100 heads of state and government are expected to attend the Rio de Janeiro summit on June 20-22, but months of talks have failed to produce an agenda for the event which is to mark the 20th anniversary of the Rio Earth Summit. \"We need world leaders to make the issues on the table at Rio+20 their own personal priority. Nothing else will do,\" Ban told reporters at the UN headquarters. Ban is battling to rescue the summit, which he warned this week must guide the world away from a \"tragic\" end. The UN Conference on Sustainable Development is taking on the vast challenge of transforming environmental, social and economic policy to cope with a world population racing past seven billion, exhausting dwindlingresources and straining cities to the limit Ban and specialist groups have however accused governments of putting national interests before the common good in months of agonizing negotiations before the Rio summit. Another three days of talks on the agenda will be held in Rio from June 13. Some diplomats say the Brazilian hosts are expected to step in to produce a tlimeline agenda from the 80 page document that the UN\'s 193 members are currently haggling over. \"I am cautiously optimistic,\" said Ban. \"I am sure that member states will act for the interests of humanity. This is a once in a generation opportunity. Once they miss this opportunity they may have to wait a very long time.\" The summit has to agre on steps to boost the so-called green economy, set out Sustainable Development Goals on areas ranging from global health and poverty to preserving oceans and natural resources, and decide how new targets should be policed.