UN climate talks on Tuesday were mired in problems as environment ministers from around the world began a four-day huddle focussed on the future of the Kyoto Protocol.

The United States played down hopes for a deal with China that, by ensuring the survival of the threatened treaty, would lead to a breakthrough.

And in an exceptional show of unity, the world's four emerging giants -- Brazil, South Africa, India and China -- said a successful outcome in Durban depended on keeping Kyoto alive with a second round of commitments.

"The Kyoto Protocol should be continued and a second commitment period is a must," China's top climate negotiator Xie Zhenhua, speaking in the name of the so-called BASIC group, said at a press conference of the four countries.

"The most important issue for us in Durban is that a clear and ratifiable decision on a KP (Kyoto Protocol) second commitment period takes place. This must happen if KP parties are really committed to addressing climate change," said India's Environment Minister Jayanthi Natarajan.

Hopes of movement were raised on Sunday when China signalled willingness -- linked with conditions -- to embrace a future legally-binding treaty on carbon emissions.

Green campaigners seized on this as a chance to remove one of the roadblocks to a deal that would save Kyoto, the only treaty that sets down legally-enforceable curbs on greenhouse gases.

But US chief delegate Todd Stern on Tuesday poured cold water on the Chinese position.

"It's not my impression that there has been any change at all in the Chinese position with respect to a legally-binding agreement," Stern told a press conference.

He said key details had to be answered in such a pact.

"It would have to cover all major parties in a full way, so that it binds with equal force for everybody, unconditionally, (with) no escape hatches in the text," he said.

The 12-day climate talks are essentially a three-corner wrangle gathering the European Union along with China and the United States, which are the world's No 1 and 2 emitters.

Kyoto is an icon for green campaigners and developing countries, which seize on it as an effective means to tame global warming and show solidarity between rich and poor nations.

But the Protocol's future is clouded.

The first round of emissions pledges under Kyoto expires next year. These promises apply only to rich countries, not developing ones, nor do they concern the United States, which boycotted Kyoto in 2001.

As a result, rich Kyoto countries are refusing to sign on for a fresh round of commitments, saying this would be unfair if far bigger emitters get off the hook.

The EU has offered to sign up for a second round of commitments, but only if it secures approval for a "roadmap" leading to a new, legally-binding pact that would encompass the big carbon polluters.

Stern downplayed the significance of "legally binding," saying it was not the "be all and end all" of solutions for climate change as its defenders suggested.

Instead, he promoted an agreement endorsed in Cancun, Mexico, last year that uses a roster of voluntary emissions curbs, by 2020, to tackle greenhouse gases.

A study by German scientists released in Durban on Tuesday said these pledges would drive Earth to warming of 3.5 degrees Celsius (6.3 degrees Fahrenheit), compared to the 2.0 C (3.6 F) UN target.