De Boer, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), said talks unfolding among senior officials here marked "the first time that people are getting down to serious negotiations" for a historic deal in Copenhagen in December 2009.

But, he warned, many positions had so far been "incredibly generic" and this problem of vagueness was especially worrying among industrialised countries.

The June 3-13 Bonn talks should issue "a very clear call on governments to start submitting their ideas on what should be the key elements of a Copenhagen outcome," said de Boer.

He warned: "Politically, if Copenhagen fails we would be in huge trouble. I think that people would then begin to question the utility of this process."

Last December, parties to the UNFCCC set down a "Bali Roadmap" of talks designed to climax in the most ambitious and complex environmental treaty ever attempted.

The post-2012 pact would succeed the current pledges made under the UNFCCC's cornerstone accord, the Kyoto Protocol.

It would commit countries to deeper curbs on the heat-trapping gases that are driving climate change.

And it would beef up the transfer of clean technology to poorer economies and strengthen financial support for those countries most at risk from water stress, rising sea levels and other damage.

But de Boer said that developing countries were still awaiting a signal from industrialised economies about what they intended to put on the table, especially "the critical ingredient" of money.

Scientists say time is running out for avoiding lasting damage to the climate system.

Politicians, startled by such warnings, have forged a consensus over the dimension of the threat.

But how to tackle this peril is bedevilled by many factors.

They include the cost of switching to a low-carbon economy, the US boycott of the Kyoto Protocol and how to coax tougher pledges from China and India, which will be the giant polluters of tomorrow.

Under one scenario sketched last year by the UN's Nobel-winning expert group, the Intergovernmental Panel on Climate Change (IPCC), industrialised nations would have slash their emissions by 25-40 percent by 2020 compared to 1990 levels to help peg warming to two degrees Celsius (3.6 degrees Fahrenheit).

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