Entering a dispute pitting the United States against the European Union and some developing nations, Ban said the overriding goal of the December 3-14 meeting was to agree to launch negotiations on a pact to succeed the current Kyoto Protocol.

Ban told more than 120 environment ministers that climate change was the "moral challenge of our generation" and said there was a "desperate urgency" to act to curb rising seas, floods, droughts, famines and extinctions of wildlife.

"The time to act is now," Ban told the ministers, split over the ground rules for agreeing to launch formal negotiations on a new long-term global treaty to limit greenhouse gas emissions, expanding the 37-nation Kyoto pact to all countries.

Washington is leading opposition at talks of any mention of scientific evidence of a need for cuts in greenhouse gases of 25 to 40 percent by 2020 below 1990 levels as part of the guidelines for negotiations.

"Practically speaking this will have to be negotiated down the road," Ban said, echoing a view given by Washington. "We have two years' time before we can conclude an international deal on this issue."

Still, he also said that countries should respect a finding by the U.N. climate panel that a range of 25-40 percent was needed to avert the worst impacts of climate change.

ROADMAP

"You need to set an agenda -- a roadmap to a more secure climate future, coupled with a tight timeline that produces a deal by 2009," he said. The United Nations wants a new pact adopted at a meeting in Copenhagen in late 2009.

The United States, supported by Japan, Canada and Australia, says that even a non-binding mention of a 25-to-40 percent range could prejudge the outcome of negotiations.

"We don't want to be pre-determining what will come out of this process," said Paula Dobriansky, U.S. Under Secretary of State for Democracy and Global Affairs.

But the European Union insisted that rich nations needed to show they were leading by example to convince developing nations, such as China and India, to start braking the rise of their surging emissions from burning fossil fuels.

"I don't need a paper from Bali that says we will just meet again next year," German Environment Minister Sigmar Gabriel said. "If you want to go a long way you need to know the starting point and where you want to go."

Ban called on all nations, including the United States, to show flexibility. He also said the threat of global warming had a "silver lining" because creative solutions could create jobs and ease poverty in developing nations from Africa to Asia.

Earlier, Australia's new Prime Minister Kevin Rudd handed formal papers to Ban ratifying the Kyoto Protocol, isolating the United States as the only rich nation without binding caps on greenhouse gas emissions under the U.N. deal stretching to 2012.

Rudd, whose Labor Party won a landslide election victory last month, said Australia was already suffering from climate change -- ranging from a drying up of rivers to disruptions to corals of the Great Barrier Reef.

"What we see today is a portent of things to come," he said.

The talks are set to wrap-up by Friday or early Saturday and traditionally annual U.N. climate meetings feature hard-bargaining and all-night sessions.

The United Nations wants a deal in place by the end of 2009 to give parliaments three years to ratify and help guide billions of dollars of investments in everything from solar panels and wind turbines to coal-fired power plants.

It took eight years for enough countries to ratify Kyoto for it to come into force in 2005, a process that was slowed in 2001 by Washington's decision not to sign up. A failure of Bali to agree to start talks would sour chances of a successor to Kyoto.

Apart from Australia, 36 Kyoto nations have promised to cut emissions by an average of 5 percent below 1990 levels by 2008-12. The United States argues Kyoto would hurt its economy and wrongly excludes 2008-12 targets for big developing nations.