RIO DE JANEIRO: Leaders from around the globe gathered Wednesday to open three days of talks at the United Nations conference on sustainable development, where a sober, unambitious mood prevailed as negotiators produced what critics called a watered-down document that makes few advances on protecting the environment.    

Negotiators worked for months to hammer out a document that many hoped would lay out clear goals on how nations could promote sustainable development making economic advances without eating up the globe's resources.

But with time running out, contentious issues like technology transfers from rich to poor nations and new financing for developing countries were set aside.

Diplomats agreed on what all call a mere beginning, a step toward a roadmap on how to embrace sustainable development at the conference dubbed ''Rio+20'',  coming two decades after the landmark 1992 Earth Summit put sustainable development on the globe's agenda.

''The future we want has gotten a little further away today. Rio+20 has turned into an epic failure.It has failed on equity, failed on ecology and failed on economy,'' said Kumi Naidoo, executive director of Greenpeace.

UN Secretary General Bank Ki-moon acknowledged the world has made little progress on environmental issues since the first Rio meeting in 1992, but said leaders are working to reverse that at the Rio+20 summit.

''Twenty years ago, the Earth Summit put sustainable development on the global agenda. Yet let me be frank: our efforts have not lived up to the measure of the challenge,'' he told delegates.

“We recognize that the old model for economic development and social advancement is broken. Rio+20 has given us a unique chance to set it right.''

Critics blasted the draft document before leaders as requiring little and using language that turns what were once demands into goals for individual nations to aspire for an increasing use of renewable energy, on protecting forests, on eradicating poverty and hunger.

French President Francois Hollande told reporters that he wasn't too excited about the summit's likely results.

He highlighted what he said were two shortcomings: the failure of the document to create an international agency for development and also the inability of negotiators to agree on additional ways of financing sustainable development, including through a tax on financial transactions.

While Hollande, Russian leader Vladimir Putin and China's Chinese Premier Wen Jiabao are among more than 100 leaders expected in Rio, there are high-level absentees: US.

President Barack Obama, Britain's David Cameron and German leader Angela Merkel are all no-shows, adding to a subdued atmosphere that the action taken in Rio isn't getting the global spotlight.