Gina McCarthy, head of the US Environmental Protection Agency (EPA), has warned state governments that it will enforce regulations to slash carbon emissions from power plants even if they choose not to cooperate.

McCarthy criticized lawmakers such as Republican Senate majority leader Mitch McConnell, who have called on state officials to ignore federal deadlines to submit plans to comply with the agency's Clean Power Plan.

"The EPA is going to regulate. Mid-summer is when the Clean Power Plan is going to be finalized," McCarthy said, adding that the agency will force states that refuse to submit compliance strategies to adopt a federal implementation plan.

The EPA is expected to propose the federal implementation plan as a default for states this summer.

A dozen states, including Kentucky, West Virginia, Indiana and Wyoming, sued the EPA in August 2014, soon after it proposed its Clean Power Plan, on grounds that the agency's use of a certain section of the Clean Air Act was illegal.

A federal Circuit Court of Appeals in Washington DC will hear arguments in the case on Apr. 16.