WASHINGTON - The Obama administration took a major step yesterday toward imposing the first federal limits on climate-changing pollution from cars, power plants and factories, declaring there was compelling scientific evidence that global warming from manmade greenhouse gases endangers Americans' health.

The announcement by the Environmental Protection Agency was clearly timed to build momentum toward an agreement at the two-week international conference on climate change that opened yesterday in Copenhagen. It signaled the administration was prepared to push ahead for significant controls in the U.S. if Congress doesn't act first on its own.

The price could be steep for both industry and consumers. The EPA finding clears the way for rules that eventually could force the sale of more fuel-efficient vehicles and require plants to install costly new equipment - at a cost of billions or even many tens of billions of dollars - or shift to other forms of energy.

No analysis has been conducted by the EPA on costs of such broad regulations, although the agency put the price tag of its proposed climate-related car rules at $60 billion, with an estimated benefit of $250 billion.

Energy prices for many Americans probably would rise, too - though yesterday's finding will have no immediate impact since regulations have yet to be written. Supporters of separate legislation in Congress argue they could craft measures that would mitigate some of those costs.

Environmentalists hailed the EPA announcement as a clear indication the United States will take steps to attack climate change even if Congress fails to act. But business groups said regulating carbon emissions through the EPA under existing clean air law would put new economic burdens on manufacturers, cost jobs and drive up energy prices.