RIO DE JANEIRO - The upcoming United Nations Conference on Sustainable Development (Rio+20), also known as the Earth Summit, entered its final preliminary lap Wednesday with the third and last preparatory committee session held, said the summit's secretary-general Sha Zukang.

Sha, UN under-secretary-general for economic and social affairs, said this final round of negotiations prior to the June 20-22 conference was at a critical "make or break" point that left diplomats little room for error.

Nearly 180 countries have sent delegates to the June 13-15 talks at Riocentro, the convention center hosting Rio+20, to determine the goals and objectives that will be presented to heads of state and government attending the summit next week.

"We need to radically accelerate the pace (of talks), we have barely three days of negotiations, days during which everything will move forward or break down," said Sha, urging envoys to "concentrate on the important issues."

"A great responsibility rests on our shoulders, we simply cannot make a mistake," Sha added.

Following the third preparatory committee session, Brazil has scheduled several debates on sustainable development issues through next Tuesday, the day before the summit begins.

Rio+20 marks the 20th anniversary of the landmark UN Conference on Environment and Development, hosted by Rio in 1992.