But no date is set for concluding the negotiation process, nor are there any pre-conditions for the talks.

Experts say Kyoto's so-called second commitment period must deliver swingeing reductions in emissions to avoid potentially crippling damage to the world's climate system by fossil-fuel gases.

The 2008 negotiations are officially a "review" of the Kyoto Protocol -- a broad assessment of what changes should be made for the treaty's next commitment period.

Any changes will then have to be negotiated in full and subsequently ratified.

Agreement came after the 12-day marathon talks in Nairobi under the 189-member UN Framework Convention on Climate Change (UNFCCC) went hours into overtime.

Developing countries had feared that any preconditions for the review meant they would be forced into accepting binding curbs on their pollution -- a requirement that would carry an economic price.

Under Kyoto's present format, only 35 industrialised nations that have signed and ratified the pact are required to make targeted cuts in emissions of greenhouse gases, including carbon dioxide (CO2).

But China, India and other high-population developing countries are fast becoming big emitters of CO2 in their own right as they voraciously burn oil, gas and coal to power their economies.

Environment ministers from industrialised countries also agreed that global emissions of greenhouse gases had to be halved, although they did not set a date.

The European Union (EU) has been pushing for a deadline of 2050, contending this will limit the rise in Earth's atmospheric temperature to 2 C (3.6 F) by 2100 compared with 1900.

"Thanks to the leadership of the European Union, we now have a solid work plan to combat emissions after 2012," said European Environment Commissioner Stavros Dimas.

"There is no time to wait. We must cut emissions by 50 percent in the next half a century to end global warming."

The present Kyoto Protocol obliges industrialised countries to reduce their emissions to a level that is around five percent below the 1990 benchmark.

For the 2013-2017 commitment period to work, a way must be found to get the big developing countries to tackle their burgeoning pollution and also tighten the screw on emissions by industrialised countries.

Another problem will be how to establish tighter cooperation with the United States, which by itself accounts for a quarter of global emissions.

Under President George W. Bush, the United States has refused to ratify and abide by the Kyoto Protocol. Even if a pro-Kyoto president takes office in the United States in January 2009, he or she is unlikely to have the political elbow room to ratify Kyoto in time for joining the negotiations for 2013-2017, diplomats in Nairobi said.

Activists say the 2013-2017 negotiations must wrap up by the end of 2009, as the outcome will take two or three years to be ratified.

Any spillover into 2010 could lead to a gap between Kyoto's commitment periods, which would badly dent confidence in the fledgling carbon markets, launched under Kyoto in 2005, said Hans Verolme, director of WWF's climate-change programme.

The Nairobi meeting also ran into fire from grass-roots advocates for poorer countries.

UNFCCC Executive Secretary Yvo de Boer, though, said developing nations would "walk away with a very significant package".

He pointed to decisions on managing and financing Kyoto's Adaptation Fund to help vulnerable countries cope with the effects of climate change and on setting a five-year work programme to identify which kind of adaptation measures would be useful.

Another area of progress was on streamlining the Protocol's Clean Development Mechanism (CDM), an incentive for transferring low-polluting technology to poor countries, so that African economies could benefit from it, said de Boer.

Kyoto became operational in February 2005 after a long and agonising gestation.

Two more countries, Sierra Leone and Lebanon, have now ratified it, bringing the total to 168, it was announced in Nairobi on Friday.

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