Brazil on Saturday took the helm of talks to forge a global deal on preserving the environment and rooting out poverty ahead of a gathering of world leaders starting in just four days.

Five months of negotiations on a vast document, due to be endorsed at the three-day summit climaxing the UN Conference on Sustainable Development, have failed to break the deadlock on several issues.

The task of coaxing out a deal fell to Brazil, as host of the "Rio+20" talks, which marks the 20th anniversary of the summit that yielded landmark agreements on climate change, desertification and biodiversity.

To speed things up, the host country came up with a consolidated text expected to be made public later Saturday.

"The Brazilians made the political decision to produce a balanced text. We must go to the heart of the deadlock," Nikhil Seth, head of the UN Sustainable Development division, told a press briefing Saturday.

Brazilian delegation chief Luiz Alberto Figueiredo said the goal was to wrap everything up by Tuesday, the eve of the summit.

"We have no intention of handing undecided issues to heads of state," he told reporters on Friday.

Conference sources say a compromise appears in sight on the divisive "green economy" concept, which would be replaced by the more palatable "green economy policies."

Figuereido said Brazilian President Dilma Rousseff might take stock of the issue at the G20 summit of leading rich and emerging powers Monday and Tuesday in Los Cabos, Mexico.

The organisers say they expect around 116 heads of state or government to show up, capping a weeklong gathering of as many as 50,000 activists, business executives and policymakers.

But many political heavy-hitters will not be there. They including US President Barack Obama, British Prime Minister David Cameron and German Chancellor Angela Merkel.

Their absence is ascribed to having to deal with pressing issues, at home including the euro crisis.

But lingering in the background are memories of the traumatic 2009 UN climate summit in Copenhagen.

This event was a near fiasco, for heads of state and government arrived at the meeting expecting to seal a historic deal, only to discover that they had to negotiate a minefield of unresolved issues.

As time ticked by to Copenhagen's close, a couple of dozen leaders cobbled together a last-minute declaration to save face -- a deal derided by greens as a sellout of the environment and by left-leaning Latin American governments as a betrayal of UN democracy.

Problems in Rio include a set of "Sustainable Development Goals" to succeed the UN's Millennium Development Goals, due to expire in 2015, how to encourage the green economy and mustering funds to promote sustainable development. Poorer countries are calling for 30 billion dollars a year.

Another area of textual friction is over how or whether to reaffirm the "Rio Principles" set down in the 1992 summit, which say countries have "common but differentiated responsibilities."

The phrase is designed to ensure that poor countries do not have to shoulder the same burden as rich countries in fixing Earth's environmental problems.

A panoply of events is unfolding in Rio alongside the political haggling, including a forum of executives discussing the benefits -- and obstacles -- of doing green business.

There is a "counter-summit" gathering indigenous peoples and eco-militants who are demanding radical change. They say the world's economic model is broken and there is no point tinkering with with it.

Hundreds of side events are showcasing issues touching on the world's many environmental ills, from climate change, deforestation, over-fishing and loss of coral reefs to the problems of slum dwelling and clogged transport systems in fast-growing economies.