On April 2 a US Supreme Court ruling affirmed a role for the federal government in setting limits on greenhouse gas emissions under a 1990 US clean-air law.

Bush said regulations should be based on his proposal to cut US petrol use by 20 per cent over the next decade, a target laid out in his State of the Union speech in January.

"We're taking action, by taking the first steps toward rules that will make our economy stronger, our environment cleaner, and our nation more secure for generations to come," he told reporters at the White House.

Federal rule-making "is an important first step ... in addressing greenhouse gas emissions from automobiles," EPA head Stephen Johnson told reporters.

Although, White House spokesman Tony Snow said that Bush remains opposed to mandatory nationwide caps on greenhouse gas emissions, such as those set by the UN Kyoto Protocol. "The market-based approach seems to have worked," Snow said. "So the president's position is still the same."