FCA US has said it was "disappointed" the US Department of Justice Environmental and Natural Resources Division (DOJ-ENRD) had filed a civil lawsuit accusing it of illegally using software to bypass emission controls in 104,000 diesel vehicles sold since 2014.

In a statement, the automaker said it had been working with the US Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) "for many months, including extensive testing of the vehicles, to clarify issues related to the company's emissions control technology in model year 2014-2016 Jeep Grand Cherokee and Ram 1500 diesel vehicles".

"FCA US is currently reviewing the complaint, but is disappointed that the DOJ-ENRD has chosen to file this lawsuit. The company intends to defend itself vigorously, particularly against any claims that [we] engaged in any deliberate scheme to install defeat devices to cheat US emissions tests."

FCA last week said it had developed updated emissions software calibrations that it believes address the concerns of EPA and CARB, and had formally filed for diesel vehicle emissions certification with the regulators for its 2017 model year Jeep Grand Cherokee and Ram 1500 diesel vehicles.

"Subject to the permission of EPA and CARB, FCA US intends to install the same modified emissions software in 2014-2016 Jeep Grand Cherokee and Ram 1500 diesel vehicles. FCA US believes this will address the agencies' concerns regarding the emissions software calibrations in those vehicles," the statement added.

"FCA expects that the installation of these updated software calibrations will improve the 2014-2016 MY vehicles' emissions performance and does not anticipate any impact on performance or fuel efficiency.

"Notwithstanding this lawsuit, the company remains committed to working cooperatively with EPA and CARB to resolve the agencies' concerns quickly and amicably."

A Reuters report said the lawsuit, filed in US District court in Detroit, is a procedural step that may ramp up pressure on Fiat Chrysler and comes amid growing scrutiny of diesels by regulators around the world.

The lawsuit could ultimately help lead to a settlement, as in an earlier probe of Volkswagen that will cost it up to US$25bn (GBP19.2bn) but which affected a much larger number of vehicles, the news agency suggested.

Reuters added the lawsuit also names Fiat Chrysler's unit VM Motori which designed the engine in question. Reuters reported last week the Justice Department and EPA had obtained internal emails and other documents written in Italian that look at engine development and emissions issues that raise significant questions. The investigation has scrutinised VM Motori, the report said.

The suit also said VM employees from Italy worked at Fiat Chrysler's Michigan headquarters on engine calibration and air emission issues.

According to Reuters, the lawsuit asserts the automaker placed undeclared "defeat devices," or auxiliary emissions controls, in 2014-2016 Fiat Chrysler diesel vehicles that led to "substantially" higher than allowable levels of nitrogen oxide, or NOx pollution, which is linked to smog formation and respiratory problems. The lawsuit also asks a court to require Fiat Chrysler to fix the vehicles and bar it from selling vehicles with excess emissions as well as unspecified civil penalties. EPA said in January the maximum fine is about US$4.6bn.

Reuters noted Fiat Chrysler faces a separate criminal investigation on the same emissions issue by the Justice Department and probes by the US Securities and Exchange Commission and many US states.