"It's unclear when and if there's going to be a global carbon market," Harlan Watson, the head of the US delegation, told a press conference in Vienna.

The European Union as well as US allies such as Australia and Canada are working on separate schemes to reduce carbon emissions and these cannot be brought together easily, Watson said.

"I think you're seeing differences ... among these schemes, so getting all of this together in one global carbon market, I think that it's going to take some time," Watson said.

Experts see carbon trading as one of the most effective ways of combating climate change, by allowing countries that pollute beyond their allowance under the Kyoto Protocol to buy carbon credits from those countries that have stayed within their target range.

Under the treaty, rich nations can also offset their carbon reduction commitments by funding initiatives in developing countries.

But Watson said "there is a fairly active voluntary carbon market in the US," in which individual companies can participate, despite the United States not being part of the Kyoto Protocol.

The United States unveiled a surprise plan in June for tackling global warming through technology, leading to concern among many European nations and environmental groups that the US initiative would conflict with the existing UN process.

More than a thousand representatives are gathering in Vienna this week as part of talks under the United Nations Framework Convention on Climate Change.

This is in preparation for a conference in Bali in December to discuss climate commitments for the period following 2012, when the Kyoto protocol expires.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.