(NBC News) - General Motors has become the sixth automaker to be accused of using some form of cheat device to pass its diesel emissions tests.

A class-action lawsuit filed Thursday claims 705,000 Chevy Silverado and GMC Sierra trucks equipped with the company's Duramax diesel emit two to five times more than the legally acceptable levels of nitrogen-oxide. The trucks impacted are 2011-2016 model year pickups.

"GM sold its Duramax line as a powerful diesel that magically emitted lesser amounts of harmful NOx pollutants," said Steve Berman, managing partner of the Hagens Berman firm, which has filed suit against other lawmakers claiming similar violations.

"But as our firm's testing revealed, GM could not accomplish what it promised consumers. Instead, it used complex devices to mask its vehicles' output of emissions, cheating emissions test, cheating purchasers, and putting all of us at risk of increased respiratory illness."

The lawsuit means GM joins Volkswagen — which admitted guilt and has paid more than $25 billion in fines and fees — Daimler AG, Subaru, Peugeot, Nissan and Fiat Chrysler in being accused of or sued for using some sort of software or hardware to get around emissions rules.

The plaintiffs are Andrei Fenner of Mountain View, California, and Joshua Herman of Sulphur, Louisiana, who claim they would not have bought their respective 2011 Sierra and 2016 Silverado trucks, or would have paid less for them, had they known about the alleged cheating, according to a Reuters report.

The law firm is the same one that targeted Volkswagen and managed to force them to pay billions of dollars in settlements and penalties. Last year, the firm targeted GM over its Cruze Diesel, charging the company with using computer software to produce lower emission readings during testing.

"We have been systematically going through diesel cars made by all manufacturers (doing independent testing) since Volkswagen's scandal came out," Berman said. "That led us to Mercedes, that led us to FCA and now we're adding GM to the list of cheaters."

GM issued a statement defending itself almost immediately after the suit was filed.

"These claims are baseless and we will vigorously defend ourselves," the maker said in a statement. "The Duramax diesel Chevrolet Silverado and GMC Sierra comply with all U.S. EPA and CARB emissions regulations."

According to the Detroit News, the 190-page complaint asserts that the environmental damage caused by GM's vehicles could surpass that of the Volkswagen's vehicles.

"This is what General Motors… promised when selling its popular Silverado and Sierra HD Vehicles — that its Duramax engines turned 'heavy diesel fuel into a fine mist,' delivering 'low emissions' that were a 'whopping reduction' compared to the prior model and at the same time produced a vehicle with "great power,'" the lawsuit states. "GM claimed its engineers had accomplished a 'remarkable reduction of diesel emissions.'"