"There is really no excuse not to act," Yvo de Boer, head of the UN Climate Change Secretatiat in Bonn, told Reuters of the mounting evidence that human activities, led by burning fossil fuels, were damaging the climate.

The third report by the UN climate panel, on Friday, said combating global warming would mean at worst a 3 percent cut in world gross domestic product in 2030. Less stringent measures could even slightly boost world growth.

"We have the instruments to deal with it in a cost-effective way," de Boer said.

The Bonn May 7-18 meeting will look at how to widen the UN's Kyoto Protocol, which binds 35 nations to cut emissions in a first phase until 2012, to include outsiders like the United States, China and India.

And the talks will prepare for annual negotiations among environment ministers in Bali, Indonesia, in December. Debate will cover issues such as helping developing nations to protect forests and promoting transfers of clean technology.

BALI

Despite public concern about climate change, some delegates say it is uncertain whether ministers will be ready to launch formal talks to work out a global pact to replace Kyoto in Bali.

Kyoto binds rich nations to cut emissions by 5 percent below 1990 levels by 2008-12. Investors say time is short because anyone building a power station, for instance, needs advance warning of new rules to make the plant conform.

The United States and the European Union agreed at a summit last week that global warming was an "urgent" priority but differed sharply about how to fix it.

The EU plans to cut emissions by at least 20 percent by 2020 and by 30 percent if other industrialised nations go along.

President George W. Bush opposes Kyoto-style caps on emissions, reckoning they will cost US jobs and that Kyoto is wrong to omit developing nations until 2012.

"There is no silver bullet," said Harlan Watson, the chief US climate negotiator. He said a broad policy mix was needed, ranging from burying carbon dioxide emitted by power plants, investments in hydrogen or nuclear power.

"The United States is making it increasingly clear that they recognise the urgency of the problem and the need for global action and yet don't want to be tied down by targets and to have the carbon markets as the sole approach," de Boer said.

The European Union says participation by the United States, the world's biggest economy and top emitter of carbon dioxide ahead of China, Russia and India, is vital to get developing nations to agree to do more.