In Brussels for talks with his European Union counterparts, James Connaughton said requirements of the 1997 protocol were simply too costly for the United States.

Specifically he dismissed as "inappropriate" the option of trading in greenhouse gas emissions, a system launched by the European Union this year as a way of encouraging companies to cut their production of carbon dioxide.

"The (US) president is pursing a strategy that is aimed at advancing economic growth, not subtracting from economic growth, and therefore emissions trading is not a useful tool," Connaughton told a small group of reporters.

The United Nations (UN) Kyoto treaty commits 39 industrial nations and territories to trim their output of six greenhouse gases -- especially carbon dioxide -- by 2012 compared with 1990 levels.

But the pact's impact has been limited because the United States opted against ratifying it. Connaughton said that targets set by Kyoto would have cost the US 500 billion dollars.

The future of climate negotiations after the Kyoto Protocol -- which is due to expire in 2012 -- will be explored at a UN conference from November 28 to December 9 in Canada.

The US official said Washington would be an "active" participant in the discussions, but noted that the Kyoto treaty itself obliges those who ratify it to start preparing for the post-2012 era.

"Those who are party to the Kyoto Protocol have already committed by the terms of the treaty to starting a discussion about what happens after 2012," he said. "From our point of view the post 2012 situation is already occurring."

Connaughton, who was notably due to meet EU energy commissioner Andris Piebalgs, was keen to underline the areas of agreement between Washington and Brussels.

"While we have disagreements over the structure of the Kyoto Protocol, the common ground that we share is much more substantial," he said.

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