CANCUN (Reuters) Saying the health of the planet is at stake, United Nations Secretary-General Ban Ki-moon urged 190 nations meeting in Mexico 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 talks which end on to December 10 talks in the Caribbean resort of Cancun where rich and poor nations are split over cutting greenhouse gas emissions.

After US President Barack Obama and other leaders failed to work out a new UN 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.

“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 UN'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,” US 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.”