The upcoming United Nations Conference on Sustainable Development (Rio+20 Earth Summit) is a valuable opportunity that should not be missed, a senior UN official said Friday.

More than 130 heads of state or government and tens of thousands of delegates will gather here to discuss sustainable development blueprint and make important decisions on the global sustainable development process, said Sha Zukang, UN under-secretary-general for economic and social affairs.

"At this moment, the world is standing at a historic starting point, and the Rio+20 provides a rare opportunity in 20 years to the countries," Sha said in a written interview with Xinhua as the preparatory committee session for the summit was being wrapped up here.

"The international community has no reason to miss this chance," said Sha, who is also the summit's secretary general.

The preparatory session was held on June 13-15 at the Riocentro Convention Center that will host the Rio+20. Nearly 180 countries have sent their delegates to the session, the last one prior to the summit, to determine the goals and objectives that will be presented to the world leaders on June 20-22.

Sha also urged the delegates to show more flexibility in the final round of negotiations and speed up the negotiation process to prepare a final document for the summit.