Existing rules require U.S. utilities and refineries to invest in state-of-the-art pollution controls if a plant undergoes a major expansion or modification.

In November, the Environmental Protection Agency proposed rules to change the definition of "routine maintenance," to give utilities more leeway to modify plants without triggering extra pollution-reduction requirements.

This week the EPA published the so-called "new source review" rules in the Federal Register, formalizing some rules and starting a time clock toward finalizing others.

But the nine states accused the administration of gutting the Clean Air Act.

"I join my colleagues in other states to challenge this assault on the Clean Air Act and continue to fight to achieve the goals that the law intended," New York Attorney General Eliot Spitzer said at a press conference.

The rule change will cause more acid rain, smog and respiratory ailments like asthma, he added.

The EPA has defended its new rules as simply giving power plants and oil refineries more flexibility to cut emissions.

The agency this week said it will prevail in the court challenge. "At the end of the day, the actions we have taken will be found to be clearly consistent with the Clean Air Act," said EPA spokesman Joe Martyak.

A batch of rules finalized this week have clear environmental benefits, Martyak said. Other regulations issued as proposed rules - including the controversial routine maintenance modifications - will be open for public comment and possible revision, he said.

But the rules were roundly criticized in November by Democratic lawmakers and environmentalists.

"The administration's rules are clearly not what Congress intended when it passed the Clean Air Act," said Sen. Hillary Clinton of New York, a Democrat.

The issue is pivotal for aging coal-fired utilities in the Midwest that could face hundreds of millions of dollars in new investments. Emissions from those plants drift over Northeast states because of wind patterns.

The Electric Reliability Coordinating Council, a utility lobbying group, called the new rules "a step in the right direction."

The National Association of Manufacturers said the new rules will bring cleaner air and boost energy supplies.

They will "provide business planners with greater certainty as they work to increase production and limit air pollution in a cost-effective manner," said Jeffrey Marks, NAM's air quality director.