The row, on the eve of a ministerial gathering of the United Nations Framework Convention on Climate Change (UNFCCC), focuses on so-called carbon sinks -- forests that would be planted to soak up carbon dioxide (CO2), the biggest greenhouse gas.

Under the UN's Kyoto Protocol on global warming, rich countries will be able to offset forest-planting in poor countries against their requirements to cut emissions of C02.

Negotiations under way at the December 1-12 UNFCCC meeting here aim at setting down the fine technical details as to how this part of the Protocol will work, such evaluating how much C02 will be stored, the kind of forest that will be planted and the minimum time the "sink" must grow before being harvested.

Diplomats said there was discord over whether these "sinks" planted abroad should only comprise species that are native to the country and whether genetically-modified trees should be allowed into the mix.

GM technology is cautiously spreading out of food crops and one conceivable outlet would be trees whose genes have been engineered to make them grow faster, thus soaking up CO2 more quickly.

However, green groups, which are a powerful force in the European Union, are angrily against any extension of genetic engineering.

Diplomats said negotiators were hoping to get a deal by late Tuesday to avoid having to send the problem on to environment ministers.

Their two-day meeting is aimed mainly on consultations and round tables on the more distant problems of climate change rather than the minutiae of Kyoto's rulebook.

Kyoto was signed as a framework agreement in 1997, but it took four years to conclude a deal on its notoriously complex rulebook and there persist even now negotiations over its finer points.

The pact was crippled by a US walkout in 2001 and now needs ratification by Russia to take effect.

If there is no agreement on the "sinks" by Friday's close, the impasse "could give further arguments" to Russia to drag its feet about ratifying the Protocol, a European source said.

Kyoto requires industrialised signatories, but not developing ones, to meet pledged reductions in output of six greenhouses gases by a deadline of 2008-2012 compared with their 1990 levels.

It offers various mechanisms as an incentive for reaching this goal, including forest planting.

Scientists say that "sinks" are at best only a temporary help for global warming unless they are managed carefully and the CO2 stored by the trees is not released.

The stored CO2 returns to the atmosphere after the tree dies and starts to rot.

SPACE.WIRE