Rich countries will have to stop the consumer high life as part of any deal to heal the world's social and environmental stresses, a top UN official said ahead of a key development summit this week.

"We don't need more cars, more TVs, more whatever" UN Development Program chief Helen Clark told AFP in an interview ahead of the Rio+20 summit starting Wednesday.

The 116 heads of state and government and their populations -- rich and poor -- face "chaos" unless the three day summit can at least lay the groundwork for economic growth that eases poverty and preserves natural resources, said the former New Zealand prime minister.

"I think there is a high level of awareness that the planet is in peril, to put it bluntly," said Clark, who will be one of the key figures at the Rio de Janeiro event.

Negotiators are struggling to get agreement on the final declaration.

Differences between rich and poor, east and west on topics such as how to define "green economy" and how to set new global development goals have bedeviled negotiations for months.

Clark insists though that every leader agrees on the key problem: how to ensure economic growth that helps the most destitute without further damaging an environment that is being "wrecked underneath our feet."

"So the issue is how to get human development that will see it continue to rise for the world's poorest people and people in developing countries. Because frankly human development in the West -- we don't need more cars, more TVs, more whatever.

"Our needs are by and large satisfied, although the recession has put a lot of strains on that."

"There is, in my opinion, a very heavy responsibility on the countries of the north to look at how they sustain their living standards with a much lower environmental footprint," Clark added.

Setting up a new index for economic progress to rival the venerable Gross Domestic Product and pressing the case for the Green Economy -- economic decision-making that takes into account the impact on the environment -- will feature highly on Clark's summit agenda.

There is a growing campaign by many governments, including from Britain's Prime Minister David Cameron, to use a wider measure for economic and human progress for key decisions.

Clark says the UN Development Program should have a new version ready for next year of its annual human development index, adding environmental sustainability factors to the equality measures already used.

The UNDP will hold an event in Rio on the need for new measures. Bhutan's Prime Minister Jigmi Thinley will discuss his "gross national happiness" index there.

Many poorer countries are suspicious of the West's demands for tighter environmental regulations in international negotiations. Clark says they see the green economy as a code for "green protectionism" that could hinder their economic growth.

Clark and UN leader Ban Ki-moon say however that moving toward a green economy can be a source of growth, much-needed jobs, investment and exports.

So do the 116 heads of government and state understand the stakes?

"Quite a number do. Often with these issues, short term politics get in the way," Clark said.

"I doubt you will have anyone come who says 'this is completely irrelevant to me' because everyone knows it is relevant."

"The world economy ain't what it was. Societies are under a lot of strain," said Clark.

"The toxic combination of falling incomes, social unrest and environmental degradation. This is reality. We have got a common problem here. We need to have a shared vision of how to tackle it.

"We are heading for chaos if we don't tackle these issues," she said.