The UN Environment Programme (UNEP) is emerging as a hot issue in preparations for June's Rio conference, styled as a once-in-a-generation chance to restore a sick planet to good health.

The US is fighting a proposal, backed according to France by least 100 countries, for transforming UNEP from a poorly noticed, second-string unit into a planetary super-agency.

Environmentalists have long complained that Nairobi-based UNEP, set up in 1972 as an office of the UN and with a membership of only 58 nations, lacks clout to deal with the globe's worsening ills.

These range from climate change, water stress and over-fishing to species loss, deforestion and ozone-layer depletion.

But the environmental mess also coincides with the crisis of capitalism, which greens say is blind to the cost for Nature in its relentless quest for growth.

The fateful intertwining of these problems points to a unique chance of a solution at the June 20-22 "Rio+20" conference, they argue.

With possibly scores of leaders in attendance, the 20-year follow-up to the famous Earth Summit has the declared aim of making growth both greener and sustainable.

"The new capitalism which emerges from the crisis has to be environmental, or it won't be new," French Ecology Minister Nathalie Kosciusko-Morizet said on Tuesday.

The key vehicle would be UNEP, which according to the vaguely-worded French proposal would be changed into the World Environment Organisation.

It would become the UN's 16th "specialised" agency alongside the World Health Organisation (WHO), Food and Agricultural Organisation (FAO) and so on.

To the outsider, this may sound at best like a bit of terminological tinkering -- at worst, just another bureaucracy-breeding machine.

Experts, though, say status change could be surprisingly far-reaching.

Specialised UN agencies have high degrees of autonomy, enabling them to set agendas, frame international norms, stir up interest in dormant issues and sometimes poke their noses into areas of national sovereignty.

At its most ambitious, a World Environment Organisation would embrace not just the member-states which fund it but also business, green and social groups, becoming a very loud voice indeed.

It could intrude into sensitive areas such as trans-border use of water resources, fishery quotas and habitat use -- and even monitor environmental standards for trade in goods and services.

According to Kosciusko-Morizet's ministry, more than 30 European countries back the French proposal, along with 54 countries in Africa, plus Thailand, Malaysia, Nepal, Chile, Uruguay and others.

But in a US presidential election year where green issues -- especially foreign ones -- are easily trumped by domestic politics, Washington has set down a marker.

"We do not believe that international efforts on the environment and sustainable development would be improved by creating a new specialised agency on the environment," a State Department official told AFP.

"We prefer to work towards a strengthened role for UNEP, as well as better coordination across the UN system in integrating environment into development, and in working towards sustainable development."

Canada, like the US, says it prefers a smarter, better-connected UNEP.

Tensions over this are now emerging at preparatory talks on the "zero draft," a document that will be finessed into June's all-important summit communique.

"The Americans have come out guns blazing," said Farooq Ullah, head of policy and advocacy at a London-based NGO called Stakeholder Forum.

"The risk, of course, is not necessarily that they would veto it (a super-UNEP) but that they would pull out their funding for it. A big part of UNEP's funding comes from the Americans, so it would be a major blow," he stressed.

Could the dispute rip Rio apart? Or could it doom it to dismal compromise, as many view the outcome of 2009 Copenhagen climate summit?

"The biggest risk with these things that have a lot of interest is that if you push too far too quickly and it becomes too contentious, it will just be negotiated out," warned Ullah.

Lucien Chabason of a French thinktank, the Institute of Sustainable Development and International Relations (IDDRI), said the outcome did not have to be dramatic.

"One can imagine a mixture of the two ideas, in which Rio adopts a position in principle to beef up UNEP and launch a negotiation process," he said.