A federal appeals court halted a challenge by 15 states to pending U.S. Environmental Protection Agency (EPA) regulations to curb greenhouse gas emissions from power plants.

A three-judge panel of the U.S. Court of Appeals for the D.C. Circuit said the rules cannot be challenged until they are finalized. The EPA is expected to finalize the regulations this summer.

"Petitioners are champing at the bit to challenge EPA's anticipated rule restricting carbon dioxide emissions from existing power plants," writes Judge Brett Kavanaugh. "They want us to do something that they candidly acknowledge we have never done before: review the legality of a proposed rule."

The states, as well as two major coal companies, had argued to the appeals court that the EPA overreached its authority. The challengers said in their lawsuit that the EPA is ignoring a provision in the Clean Air Act that prohibits the agency from regulating a pollutant from a power plant if other types of pollutants from the plants are already being regulated.

The states argue that when the law was amended in 1990, Congress barred the EPA from regulating the same pollution source under the section of the law being used in the pending rule, if that source is already being regulated under a different part of the law. Therefore, because the EPA is already restricting mercury and other hazardous emissions from power plants, they argue, the agency can't now restrict greenhouse gas emissions from the same plants.

The EPA says the restriction in the law is against regulating the same pollutant twice, not regulating different pollutants from the same source.