The top US official at the United Nations climate talks in Nairobi said the proposed tax hit at the principle that countries could take a differentiated approach to tackling global warming, whether members of the Kyoto Protocol or not.

"We would not see in this case this kind of approach as being the most constructive one or the most effective one," said the official, Paula Dobriansky, who is undersecretary of state for democracy and global affairs.

The tax idea was sketched out on Monday by French Prime Minister Dominique de Villepin.

He said France would present its European Union partners with proposals early next year for a "carbon tax" on imports of industrial goods from countries that "refused to commit themselves in favour of the Kyoto Protocol after 2012."

The proposal did not feature in a speech by French President Jacques Chirac that was read to the Nairobi conference on Wednesday by Environment Minister Nelly Olin.

The Kyoto Protocol commits industrialised countries that have signed and ratified it to trim their climate-threatening carbon pollution by a deadline of 2012, as compared with a 1990 benchmark.

President George W. Bush walked away from Kyoto in 2001, saying he would never submit the treaty to ratification by the US Senate as, in his view, it would be too costly for the US to implement.

Bush also argued that the treaty was unfair, as it did not include China, India and other fast-growing developing countries in the roster of countries that have to make targeted pledges on cutting or containing their emissions.

The present pledges for Kyoto run out in 2012, and negotiations are due to start next year on commitments beyond this date.

Australia is the only other Kyoto holdout, and Villepin's proposal ran into heavy flak from sections of the Australian media, which branded the tax idea as protectionist.

The EU commissioner for the environment, Stavros Dimas, said the idea of a carbon tax on imports from non-Kyoto countries had previously been discussed within the EU. A "detailed study" is already under way to explore the benefits, impacts and feasibility of such an initiative, he said.

An EU source, speaking to AFP on condition on anonymity, said the tax proposal was politically sensitive, given the need to avoid conflict with the rules of the World Trade Organisation (WTO).

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