Talking to reporters Tuesday, McClellan said the administration of President George W. Bush had made an unprecedented commitment to trim the growth of greenhouse gases without moving to curtail America's economic growth.

"We are continuing to move forward in an aggressive way to address climate change. It is a serious matter; it's a matter that this administration takes very seriously," he said.

"Under this administration we have made an unprecedented commitment to reduce the growth of greenhouse gas emissions in a way that continues to grow our economy.

"There's a lot that we are still learning about the science of climate change, but this administration is working to advance that science and to learn more about climate change, itself, and its effect on the world."

The landmark Kyoto Protocol, which hopes to slow down global warming, goes into effect Wednesday with most of the industrialized world committed to slash gas emissions but the United States and Australia holding out.

The treaty, which comes into force at 0500 GMT, seeks for the industrial world as a whole to slash its greenhouse gas emission by 5.2 percent by 2012, with targets set according to each country's pollution level.

Thirty inductrialized countries are among the 141 countries which have signed the treaty, but not the United States or Australia, which say that Kyoto's burden to their economies would be too great.

McClellan said Washington was moving forward on a number of environmental iniatives including 3.6 billion dollars in tax incentives for renewable energy effecient technologies over the next five years.

He added the adminstration was also "increasing fuel economy standards."

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