* U.N. climate talks agree deal on global warming
* Vulnerable states say accords leave them vulnerable
* Number of key issues deferred until 2012
By Nina Chestney and Jon Herskovitz
DURBAN, Dec 11 (Reuters) - Countries from around the globe agreed on Sunday to forge a new deal forcing all the biggest polluters for the first time to limit greenhouse gas emissions, but critics said the plan was too timid to slow global warming.
A package of accords agreed after marathon U.N. talks in South Africa extended the 1997 Kyoto Protocol - the only global pact enforcing carbon cuts - allowing five more years to finalise a wider pact which has so far eluded negotiators.
Kyoto's first phase - due to expire at the end of next year but now extended until 2017 - imposed limits only on developed countries, not emerging giants like China and India. The United States never ratified it.
Those three countries and the EU held a last-ditch huddle in the conference centre before finally agreeing to wording that commits them to a pact with legal force, although exactly what form it will take was left vague.
Countries also agreed the format of a fund to help poor nations tackle climate change.
But many small island states and developing nations at risk of being swamped by rising sea levels and extreme weather said the deal marked the lowest common denominator possible and lacked the ambition needed to ensure their survival.
Agreement on the package, reached in the early hours of Sunday, avoided a collapse of two weeks of climate talks and spared the blushes of host South Africa, whose stewardship of the fractious negotiations came under fire from rich and poor nations.
"We came here with plan A, and we have concluded this meeting with plan A to save one planet for the future of our children and our grandchildren to come," said South African Foreign Minister Maite Nkoana-Mashabane, who chaired the talks.
"We have made history," she said, bringing the hammer down on the Durban conference, the longest in two decades of U.N. climate negotiations.
Delegates agreed to start work next year on a new, legally binding accord to cut greenhouse gases, to be decided by 2015 and to come into force by 2020.
The process for doing so, called the Durban Platform for Enhanced Action, would "develop a new protocol, another legal instrument or agreed outcome with legal force" that would be applicable under the U.N. climate convention.
That phrasing was used by all parties to claim victory.
Britain's Energy and Climate Secretary Chris Huhne said the result was "a great success for European diplomacy".
"We've managed to bring the major emitters like the U.S., India and China into a roadmap which will secure an overarching global deal," he said.
U.S. climate envoy Todd Stern said Washington was satisfied with the outcome: "We got the kind of symmetry that we had been focused on since the beginning of the Obama administration. This had all the elements that we were looking for."
Yet U.N. climate chief Christiana Figueres acknowledged the final wording on the legal form a future deal was ambiguous: "What that means has yet to be decided."
Environmentalists said governments wasted valuable time by focusing on a handful of specific words in the negotiating text, and failed to raise emissions cuts to a level high enough to reduce global warming.
Sunday's deal follows years of failed attempts to impose legally-binding, international cuts on emerging polluters, such as China and India, as well as rich nations. Poor countries argue they should deserve leeway to catch up in development.
Sunday's deal extends Kyoto until the end of 2017, ensuring there is no gap between commitment periods. EU delegates said lawyers would have to reconcile those dates with existing EU legislation.
India's Environment Minister Jayanthi Natarajan, who gave an impassioned speech to the conference denouncing what she said was unfair pressure on Delhi to compromise, said her country had only reluctantly agreed to the accord.
"We've had very intense discussions. We were not happy with reopening the text but in the spirit of flexibility and accommodation shown by all, we have shown our flexibility... we agree to adopt it," she said.
Small island states in the front line of climate change, said they had gone along with a deal but only because a collapse of the talks was of no help to their vulnerable nations.
"I would have wanted to get more, but at least we have something to work with. All is not lost yet," said Selwin Hart, chief negotiator on finance for the coalition of small states.
Tosi Mpanu-Mpanu, head of the Africa Group, added: "It's a middle ground, we meet mid-way. Of course we are not completely happy about the outcome, it lacks balance, but we believe it is starting to go into the right direction."
U.N. reports released in the last month said delays on a global agreement to cut greenhouse gas emissions will make it harder to keep the average rise to within 2 degrees Celsius over the next century.
"It's certainly not the deal the planet needs - such a deal would have delivered much greater ambition on both emissions reductions and finance," said Alden Meyer of the Union of Concerned Scientists.
"Producing a new treaty by 2015 that is both ambitious and fair will take a mix of tough bargaining and a more collaborative spirit than we saw in the Durban conference centre these past two weeks." (Additional reporting by Barbara Lewis, Agnieszka Flak, Andrew Allan, Michael Szabo and Stian Reklev; editing by Jon Boyle and Peter Graff)