A unit of energy giant Edison International (EIX.N) on Thursday was charged with violating U.S. clean air laws at its coal-fired power plants in Illinois by releasing massive amounts of pollutants into the atmosphere.

Midwest Generation LLC was accused of modifying six of its plants without the required pollution control equipment and faces potentially millions of dollars in fines as well as being required to install new equipment at the plants to control emissions.

The excess illegal emissions resulting from the violations alleged in the complaint are sufficient to cause serious harm to human health and the environment, said John Cruden, acting assistant attorney general for the Justice Department's environment and natural resources division.

The complaint, filed by the Justice Department on behalf of the state of Illinois and the Environmental Protection Agency, accuses the company of illegally emitting massive amounts of sulfur dioxide, nitrogen oxide and particulate matter dating back to the 1990s.

The company said in a statement it was open to settlement talks with the government and that it already had agreed with the state of Illinois to operate under strict pollution control regulations.

While we remain open to exploring settlement of this complaint, we have a progressive record of environmental performance and leadership that we will be prepared to vigorously present and defend, the statement said.

Company spokesman Doug McFarlan said that most of the violations predate its ownership of the plants, which were acquired in late 1999 from Commonwealth Edison.

The complaint was the latest in a series of Clean Air Act cases brought by the government against coal-fired power plant operators.

The government agencies said coal-fired power plants produce more pollution than any other source in the country and the emissions can cause heart attacks, respiratory disease and premature death.

The case is USA and Illinois v. Midwest Generation LLC, 09-cv-05277, and was filed in the U.S. District Court for the Northern District of Illinois. (Reporting by Jeremy Pelofsky; Editing by Walter Bagley)