French President Nicolas Sarkozy and German Chancellor Angela Merkel wrote to UN Secretary General Ban Ki-moon arguing that states that fail to back a deal at a climate summit in Copenhagen in December should be held accountable.

"It would be unacceptable for the efforts of the most ambitious countries to be undermined by the carbon emissions accused by lack of or insufficient action by other countries," reads the letter released by the French presidency.

"For that reason, it should be possible to put in place appropriate adjustment measures targeting the countries that do not implement or fail to support this accord," they wrote.

The two leaders also pleaded for the creation of a world environmental agency, with as a first step the emergence of new institutions that would "encourage the emergence of a body of international environmental law."

The December 7-18 meeting under the UN Framework Convention on Climate Change (UNFCCC) aims to set down action for tackling heat-trapping carbon emissions beyond 2012, when the provisions of the Kyoto Protocol run out.

Representatives of the world's 17 biggest carbon polluters kicked off a week of high-stakes talks on climate change Thursday with a discussion aimed at bridging differences ahead of the Copenhagen talks.

Sarkozy, whose government is to introduce a carbon levy on domestic fuel emissions in 2010, has repeatedly argued for a European Union carbon tax on imports from regions with poor environmental standards.

Germany had yet to come out in favour of a carbon tax on imports, which a German minister has warned could be perceived by developing nations as a form of "eco-imperialism."

The European Union prides itself on being at the forefront of the climate fight.

But developing countries such as India and China argue rich countries ought to shoulder the main responsibility for mitigating global warming as they have historically emitted most of the greenhouse gases at the root of the problem.

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