WASHINGTON - The Environmental Protection Agency has not decided yet whether to regulate carbon dioxide emissions from coal-fired power plants alongside the car and truck emissions it will limit, the agency's top official said Thursday.

The EPA came under fire from congressional Democrats for moving too slowly in the wake of a Supreme Court ruling in April that carbon dioxide does qualify as a pollutant. They also slammed the agency for continuing to approve coal-fired plants without any carbon dioxide emission controls.

The determination whether to regulate carbon emissions from power plants remains under review, EPA Administrator Stephen Johnson said at a hearing of the House Oversight and Government Reform Committee.

"It's very much being considered as part of the agency's deliberative process," he testified.

The Supreme Court ruled that carbon dioxide from mobile sources like cars and trucks is considered a pollutant. That gave the EPA the authority it would need to regulate the greenhouse gas emissions from new motor vehicles under the Clean Air Act.

As a result, the EPA will propose regulations on those emissions by the end of the year, with a final decision in place by the end of 2008. Johnson said it is up to him to make needed findings and issue regulations before an air pollutant can be considered a "regulated pollutant."

The agency is studying whether the Supreme Court decision also has an impact on stationary sources of carbon dioxide emissions including coal-fired power plants, Johnson said.

"I cannot and will not prejudge what we are planning to propose," he said. "We're very much in a pre-decisional mode."

As for the dozens of new coal-fired plants already in the works, Johnson said permits will be evaluated case by case according to current law. In August, after the Supreme Court decision, the EPA granted a permit to a new coal-fired power plant, the Deseret plant in Utah, without imposing limits on carbon dioxide.

Several members of Congress and another witness argued that EPA should delay making any decision on proposed plants until the new regulations are in place.

Committee Chairman Henry Waxman, D-Calif., said he will introduce legislation to stop the EPA from approving permits for plants without controlling carbon dioxide emissions. Otherwise the energy industry may be encouraged to quickly build "dirty plants," he said.

Waxman said the administration's policy of approving massive new sources of uncontrolled carbon dioxide emission is "the climate equivalent of pouring gasoline on a fire" and called it irresponsible, indefensible and illegal.

Johnson said that while the agency needs to act with urgency on climate change, it must also be deliberative. He said the Clean Air Act is very complicated and that carbon dioxide must become a "regulated pollutant" before the agency has any authority to impose limits for greenhouse gas emissions on power plants.

David Doniger of the Natural Resources Defense Council said a dozen plants could be approved in the next 14 months without carbon dioxide emission controls, meaning emissions would continue for the 50- to 60-year life of the plants. He said there is no schedule from the EPA for dealing with power plant emissions.

No new plants should be allowed without carbon capture and storage technologies, Doniger added. He believes the EPA already has enough authority under the Clean Air Act to require that, as carbon dioxide has been ruled an air pollutant.