UN chief Ban Ki-moon warned climate talks on Tuesday that failure to overcome deadlock placed the world in peril, and begged countries to spare the endangered Kyoto Protocol.

"Without exaggeration, we can say: the future of our planet is at stake -- people's lives, the health of the global economy, the very survival of some nations," Ban told environment ministers at the start of a four-day meeting.

The conference under UN Framework Convention on Climate Change (UNFCCC) has until Friday to determine the fate of the Kyoto Protocol, the only legally binding treaty for curbing dangerous greenhouse gases.

A sense of foreboding sharpened as the 12-day climate marathon kicked into higher gear.

One veteran observer from a US NGO said he did not rule out a re-run of the Copenhagen Summit of 2009, where squabbling almost destroyed the UN climate process.

Plunging into the debate, Ban called for Kyoto to be kept alive and scheduled meetings on Wednesday with the big players.

"I urge you to carefully consider a second commitment period under the Kyoto Protocol," Ban said.

"In the absence of a global binding climate agreement, the Kyoto Protocol is the closest we have," he said.

"While Kyoto alone will not solve today's climate problems, it is a foundation to build on with important institutions. It provides the framework that markets sorely need. ... It is important that we do not create a vacuum."

Kyoto's death would leave a toxic legacy among developing countries, which see the treaty as a totem of solidarity between rich and poor, and leave the UNFCCC with only a voluntary approach for taming carbon emissions.

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

Rich Kyoto countries are refusing to sign on for fresh commitments, saying this would be unfair if far bigger emitters, accounting for more than half of all carbon pollution, get off the hook.

Chances of a deal lie in a proposal made by the European Union, which has offered to sign up for a second round of commitments in return for a "roadmap" to a new, legally binding pact encompassing the big carbon polluters, notably China and the United States.

Hopes of movement were raised on Sunday when China signalled willingness -- linked with conditions -- to embrace a future binding accord after 2020.

But on Tuesday, optimism faded.

"It's not my impression that there has been any change at all in the Chinese position with respect to a legally binding agreement," said US chief delegate Todd Stern.

Stern said the United States had conditions of its own for any such pact, and these still had to be answered.

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

In a show of unity, the world's four emerging giants -- Brazil, South Africa, India and China -- said keeping Kyoto alive was essential.

"The Kyoto Protocol should be continued and a second commitment period is a must," China's top climate negotiator Xie Zhenhua, speaking for the so-called BASIC group, told journalists.

"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.

Europe warned the big emitters that they faced the verdict of history if they threw away the chance of a deal.

"Some parties are not ready now. But if they will not even accept to commit in a foreseeable future, those parties take on an unbearable responsibility," said EU climate commissioner Connie Hedegaard.

Stern pushed for UNFCCC parties to implement an approach crafted in the final desperate hours of Copenhagen and endorsed in Cancun, Mexico, last year.

This tack is based on a voluntary register of domestic actions to tackle greenhouse gases by 2020. It would also set up a fund, worth up to 100 billion dollars a year, to help poor countries.

But a study by German scientists released in Durban on Tuesday said the current carbon pledges would lead to warming of 3.5 degrees Celsius (6.3 degrees Fahrenheit), compared to the UN's 2.0 C (3.6 F) target.

Climate researchers say a 3.5 C (6.3 F) scenario would be very bleak, dooming tens of millions of people to worse droughts, floods, storms and rising seas.