"So the idea that ... we don't understand the arguments, or we're not contemplating or taking seriously the arguments about carbon caps, of course we are," he said.

While many environmentalists have urged mandatory caps on carbon dioxide emissions, as imposed in Europe, Bush opposes the idea and advocates the development of new technologies to reduce dependence on oil.

"I would point out that ... there is a carbon cap system in place in Europe, we are doing a better job of reducing emissions here," Snow said.

The White House said Snow was referring to figures from the International Energy Agency that from 2000 to 2004, US carbon dioxide emissions from fossil fuel combustion grew by 1.7 percent, while in the European Union such emissions grew by 5 percent.

Snow said Bush had acknowledged a link between climate change and human activity and had pursued the "most aggressive program of research and technology ever" on that issue.

The United States has also been providing technology to the developing world, which is not included in the Kyoto Protocol that binds 35 industrial countries to cutting carbon dioxide emissions by at least 5 percent below 1990 levels by 2012. The United States is not bound by Kyoto targets.

The UN climate panel issued its strongest warning in a report last week that human activities, like burning fossil fuels in vehicles and power plants, were resulting in global warming.

In response, US officials played down the country's contribution to climate change, although the United States is responsible for one-quarter of the world's carbon dioxide emissions and uses one-quarter of the world's crude oil.