* Ban says "perfect is the enemy of good"
* Rich-poor split over future of Kyoto Protocol (Recasts, updates with quotes, details)
By Gerard Wynn and Timothy Gardner
CANCUN, Mexico, Dec 7 (Reuters) - Saying the health of the planet is at stake, United Nations Secretary-General Ban Ki-moon urged 190 nations meeting in Mexico on Tuesday to agree to steps to fight climate change without holding out for a perfect deal.
"We cannot let the perfect be the enemy of the good," Ban told a first session of environment ministers at the Nov. 29 to Dec. 10 talks in the Caribbean resort of Cancun where rich and poor nations are split over cutting greenhouse gas emissions.
After U.S. President Barack Obama and other leaders failed to work out a new U.N. climate treaty at a 2009 summit in Copenhagen, Ban repeatedly stressed lower ambitions for the Cancun talks despite calls by some nations for radical action.
He said the fight against global warming was "a marathon race, not a sprint" and that it was vital to start taking steps to avert floods, droughts, desertification and rising sea levels rather than insist on an all-encompassing deal.
Ministers in Cancun are seeking a package deal to set up a fund to oversee climate aid, ways to slow deforestation, steps to help poor countries adapt to climate change and a mechanism to share clean technologies such as wind and solar power.
Ban raised the pressure, telling ministers "the stability of the global economy, the well-being of your citizens, the health of our planet, all this and more depend on you."
Bolivia has led calls by some poor nations for deep cuts in greenhouse gases by rich nations in a drastic overhaul of the world economy to help protect "Mother Earth."
Many rich countries want emerging economies, led by fast-growing China and India, to do far more, including greater oversight of their programs to rein in greenhouse gas emissions.
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"We must negotiate with a certainty that we cannot waste time. We must act now," said Mexican President Felipe Calderon.
Developed and developing countries are most split about the future of the U.N.'s 1997 Kyoto Protocol, which obliges almost 40 rich nations to cut emissions by an average of 5.2 percent below 1990 levels in the five-year period 2008-12.
"The Kyoto Protocol issue continues to be very tough. It's not clear whether it's resolvable," U.S. climate envoy Todd Stern told a news conference.
"It's taking up a lot of negotiator time, energy, and focus of people in senior levels and there is an uncertainty about it that affects the general tenor and mood of the conference."
The United States is the only rich nation outside Kyoto after arguing that treaty wrongly omitted targets for 2012 for developing nations and would cause U.S. jobs losses.
Japan, Russia and Canada have been adamant that they will not approve an extension to Kyoto when the first period runs out in 2012. They want a new, broader treaty that will also bind the United States and emerging powers like China and India to act.
But developing states say rich nations have emitted most greenhouse gases since the Industrial Revolution and must extend Kyoto before poor countries sign up for action.
Christiana Figueres, head of the U.N. Climate Change Secretariat, said positions were "diametrically opposed" and the future of Kyoto was not due to be decided in Cancun.
"Germans have a wonderful word 'yein' which means both 'yes' and 'no' and I think that's the kind of attitude countries are now engaged in," she said. (Writing by Alister Doyle, editing by Christopher Wilson) For Reuters latest environment blogs, click on: http://blogs.reuters.com/environment/
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