Many experts say 2009 is the latest practical date to agree a climate pact to succeed the Kyoto Protocol beyond 2012. Any firm building a coal-fired power plant or a wind farm needs to know rules for greenhouse gas emissions years in advance.

"So finalising things in 2009 would be ideal. But we also have to be realistic about the amount of work that needs to be done," de Boer, head of the UN Climate Change Secretariat, said.

About 1,000 delegates are meeting in Vienna from Aug. 27-31 to review ways to slow warming.

And 2009 has become a matter of prestige for the United States and other rich nations in the Group of Eight.

They agreed in June that they wanted agreement by the end of 2009 on a long-term UN plan to fight global warming, partly in response to warnings of ever more floods, droughts, heatwaves and rising sea levels.

"We managed to negotiate Kyoto in two years. This is a lot more complicated," de Boer said.

The UN's Kyoto Protocol, negotiated from 1995 to 1997, binds 35 industrial nations to cut greenhouse gas emissions by five percent below 1990 levels by 2008-12. Most of the emissions are gases released by burning fossil fuels.

"For the time being 2009 is what we should be working towards," de Boer said when asked if talks might slip to 2010.

Many governments want environment ministers, who will meet in Bali, Indonesia, in December, to launch two-year negotiations to agree a broader international treaty to replace Kyoto.

A new pact would seek to involve the United States, the top emitter of greenhouse gases which is outside Kyoto, and get developing nations such as China and India to do more to brake their sharply rising emissions.