General Motors will pay a penalty of almost $146 million and forfeit emissions credits to settle allegations that nearly 6 million of its vehicles released more pollution than the automaker reported, federal officials said in a statement on Wednesday.

The Environmental Protection Agency found certain 2012-2018 model year GM vehicles were emitting more than 10% higher carbon dioxide on average than first claimed in the company's compliance reports, the EPA stated. The impacted vehicles include about 4.6 million full-size pickup and sport-utility vehicles and roughly 1.3 million mid-size SUVs, such as Chevrolet Equinox, Tahoe and Silverado models.

"EPA's vehicle standards depend on strong oversight in order to deliver public health benefits in the real world," EPA Administrator Michael Regan said in the statement. "Our investigation has achieved accountability and upholds an important program that's reducing air pollution and protecting communities across the country."

GM also agreed to a fine of $145.8 million to resolve fuel-efficiency compliance issues resulting from the EPA's investigation, a spokesperson for the National Highway Traffic Safety Administration said. 

The automaker will also cancel 50 million metric tons of greenhouse gas credits from the EPA, along with about 30.6 million gas mileage credits from the NHTSA, according to the agencies. 

The excess emissions were detected as part of mandatory testing designed to make sure vehicles are not violating federal tailpipe standards.

The White House in March finalized the toughest limits yet on climate-warming emissions from passenger cars and light trucks, part of an effort to speed up the country's less-than-smooth transition to electric vehicles. 

GM did not immediately respond to a request for comment.