US authorities said Monday they were suing Volkswagen over emissions-cheating software found in nearly 600,000 vehicles sold in the United States alone.

The Justice Department and the Environmental Protection Agency filed a civil complaint against the Wolfsburg-based auto maker in Detroit's US District Court.

The lawsuit is based on the assumption that Volswagen illegally installed software designed to thwart US emissions tests in the lab, with that software switching off to show better results in real-world driving conditions.

Huge violations of environmental laws

US authorites said the cheating software resulted in harmful nitrogen oxide emissions at up to 40 times federal environmental standards.

VW had admitted in September of last year that the cheating software was included in its diesel cars sold since the 2009 model year.

The US Justice Department said Monday the German carmaker could still face separate criminal charges. VW has been negotiating a massive recall with US regulators and is bracing for huge litigation and compensation costs.

A Volkswagen spokesman told AFP news agency Monday management would carefully study the statement of claim from the US Justice Department, adding the company would continue to cooperate closely with US authorities.

hg/nz (AP, AFP, dpa)