Countries most vulnerable to climate change are alarmed by recent proposals from rich and major emerging economies to delay a global deal to curb greenhouse gases until at least 2020, delegates and analysts at UN talks opening Monday say.

Tasked with forging a plan to stop the juggernaut of climate change, the world's nations were gathering against a backdrop of new evidence that its pace has quickened, its impact deepened.

But the 12-day talks under the UN Framework Convention on Climate Change (UNFCCC), still fragile after the near collapse of the 2009 Copenhagen Summit, are riven with conflict, participants say.

The margin for progress is narrow, and could disappear altogether unless the 194-nations body finds a way to cut through a Gordian knot of discord that has only tightened in recent weeks.

At issue is how quickly major emitters of greenhouse gases -- the United States, China, India, Brazil, the European Union and a dozen other countries that account for 80 percent of CO2 emissions -- should step up the transition towards a low-carbon economy.

Recent science shows that the window of opportunity for capping the rise in global temperatures at two degrees Celsius, the threshold for dangerous global warming, is fast closing.

Within five years, it could slam shut, according to the International Energy Agency (IEA).

The only binding treaty to curb CO2 emissions, the Kyoto Protocol, has meanwhile withered to the point where only the European Union -- which accounts for 11 percent of global emissions -- is considering new pledges after the first round expires at the end of next year.

Washington has made separate, voluntary pledges under another track of the complex UNFCCC process, while number one carbon polluter China has set national goals for using energy more efficiently. Other rich and major emerging countries have done likewise.

But their announced efforts are barely half of what's needed to give the planet a fighting chance of not busting through the 2.0 C barrier into a world ravaged by more deadly droughts, heatwaves, storms and wildfires.

Which is why, when it became apparent in the run up to the conference that top emitters -- both rich nations and the so-called BASIC group of emerging economies -- preferred to put off any new binding pledges on emissions for at least a decade, some poorer nations become alarmed.

"The push by the world's biggest carbon polluters to delay flies in the face of the overwhelming evidence in support of immediate action and represents a betrayal of the people most vulnerable to climate change and the world," said Grenada's Dessima Williams, chairwoman of the Association of Small Island States (AOSIS.)

"To fulfill our moral and ethical obligation to protect our people, AOSIS will here in Durban reject any outcome that cannot ultimately safeguard our livelihoods and guarantee the survival of our nations."

A European initiative, somewhat more ambitious, calls for a "Durban roadmap" towards the crafting of a comprehensive new climate deal by 2015, with implementation by 2020.

Negotiators and analysts said the new timeframes could cause the UN process to seize up.

"It is headed towards a real impasse in Durban, frankly, there is no way to gloss over it," said one veteran observer participating in the talks.

"There are very few options left open to wring much out of the meeting unless the position of these major countries softens considerably."

Both rich nations -- the US, Canada and Japan -- and emerging economies such as India and China have formed "a surprisingly convergent viewpoint," he added.

The UN's top climate official on Sunday acknowledged that keeping the Protocol from slipping into irrelevance while at the same time boosting the mitigation efforts of countries that are exempt from its terms, or refuse to renew Kyoto vows, will be a "tall order for governments."

But she was more upbeat on the prospects for salvaging at least a lifeline for Kyoto, while making incremental progress on a package of measures outlined at last years climate meeting in Cancun, Mexico.

These include a Green Climate Fund that will, by 2020, distribute at least 100 billion dollars annually to help poorer countries fight with climate change and cope with its impacts.

Other measures include a mechanism for technology transfer, for adaptation, and to monitor and verify national emissions reductions schemes.

A breakthrough -- if there is one -- will come in the last days of the November 28-December 9 talks when ministers arrive in Durban to try to push through a final compromise, she said.