The fate of the only international agreement that sets binding targets for curbing greenhouse gases is hanging by a thread, say veteran watchers of the UN talks unfolding here.

Protesters demonstrate against the possible end of the Kyoto Protocol during the United Nations Climate Change Conference in Copenhagen in 2009.

Failure to prolong the Kyoto Protocol\'s roster of pledges beyond 2012 would mark a perilous new low for climate negotiations and their UN architecture, set down by the 1992 Rio summit, they say.

"The collapse of the Kyoto Protocol is a plausible scenario," said Elliot Diringer, vice president of the Pew Center on Global Climate Change, a Washington-based thinktank.

"Parties are facing a choice of limited progress or no progress. If they opt for the latter, it will leave the process in a shambles."

New talks under the UN Framework Convention on Climate Change (UNFCCC), ending in Bonn on Friday, aim at building consensus for the 194-nation forum\'s next high-level meeting, running November 28-December 9 in Durban, South Africa.

Agreed in skeletal form in 1997 and implemented in 2005 after agonising talks over its rulebook, Kyoto commits 37 advanced economies to trim six greenhouse gases by an overall five percent by a 2008-2012 timeframe compared to 1990.

Overstepping a national target carries a penalty in lower emissions in the next commitment period. Any shortfall carries over to the second pledge and is multiplied by 30 percent.

Washington was one of the chief architects of the protocol but never ratified the treaty.

Former president George W. Bush said Kyoto was fatally flawed because it does not require developing giants, already major polluters, to take on similar constraints.

European countries are generally on track for their emissions reductions, but Canada is poised to miss its target by a wide margin.

At the same time, emissions by China, India, Indonesia and Brazil have rocketed -- nations bound by Kyoto account for less than 30 percent of global CO2 emissions, which hit record levels in 2010.

Even so, the protocol exerts tremendous force among poorer countries, which say it enshrines the responsibility of rich nations for unleashing the carbon demon.

"Developing countries have put a high priority on keeping Kyoto going," notes Alden Meyer of the Washington-based Union of Concerned Scientists. "It\'s the only agreement that does have binding commitments."

It also contains carefully-elaborated mechanisms for accounting and verification of emissions reductions that all parties agree should not be simply tossed aside.

There are broadly three possible outcomes for Kyoto, say experts.

The least plausible is that the Protocol\'s rich parties, the so-called Annex 1 nations, sign up for another five-year tour of duty with the same degree of legal constraints.

Japan and Russia have already bluntly said they will not do so.

At the other extreme, the threat of terminal deadlock looms larger.

"If they don\'t reach an agreement and the whole thing stalemates, it risks blocking progress on the other track of the negotiations," said Meyer, referring to breakthroughs made last December when ministers met in Cancun, Mexico.

These include steps toward a "green fund" for developing countries that could reach 100 billion dollars a year, a framework for monitoring national schemes to reduce emissions and transfer of clean technology.

The middle scenario is a stop-gap "political" agreement, in which there might be, for example, a three-year extension of Kyoto promises to secure deals in the other track.

But the European Union (EU) has warned tetchily that its backing for a Kyoto 2 should not be taken for granted.

"There is the impression that the EU will easily move into a second commitment period, that it is a foregone conclusion. That is not the case," Artur Runge-Metzger, the chief negotiator, said coming into the Bonn meeting.

Unless major emerging economies such as China, India and Brazil "make a gesture," the EU is unlikely to renew its Kyoto vows, said one European negotiator Tuesday, asking not be named.

"Ultimately, Kyoto is the only concrete decision that we can expect in Durban. It will be a defining event," said Laurence Tubiana, director of the Institute of Sustainable Development and International Relations (IDDRI).