FILE - The FIAT 500x model is seen during the official presentation in Balocco, northern Italy, Nov. 11, 2014.
FILE - The FIAT 500x model is seen during the official presentation in Balocco, northern Italy, Nov. 11, 2014.

BERLIN - German environmental lobby group DUH stepped up its campaign against auto industry pollution on Tuesday, accusing Fiat Chrysler Automobiles' 500X compact sport-utility vehicle of releasing excessive levels of toxic diesel emissions.

DUH has already accused General Motors' Opel division, Renault and Daimler's Mercedes-Benz brand of violating emissions levels. At a press conference in Berlin, DUH turned to Fiat Chrysler Automobiles (FCA).

"The extreme overruns of NOx emissions that have meanwhile been detected with an Opel Zafira, a Renault Espace, a Mercedes C-Class and now a Fiat SUV are technically not plausible and point towards defeat devices," campaigner Axel Friedrich said.

FCA declined comment.

The carmakers deny they are acting illegally, but acknowledge that some models produce higher levels of pollution when tested in conditions which are more akin to real-world driving rather than according to the norms set out in a specific test regime prescribed by regulators.

European lawmakers earlier this month backed more stringent testing methods which cut overall emissions but under some circumstances still allow a 50 percent overshoot of the legal ceiling for nitrogen oxide of 80 milligrams/kilometre.

European regulators and authorities are investigating various carmakers. But so far only Volkswagen has been accused of illegal behavior after it admitted to using software to manipulate emissions test results in a laboratory.

On Tuesday, citing tests carried out by the University of Applied Sciences in Bern, Switzerland, DUH Managing Director Juergen Resch told a news conference that a Fiat 500X, a 2015 model of the latest Euro 6 diesel generation, had released nitrogen oxide emissions that were between 11 and 22 times the legal European limits when tested with a warm engine.

The Fiat 500X released excessive emissions in all eight tests carried out with a warm engine on the dynamometer, DUH said.

The model exceeded the statutory 80 milligrams limit only during two tests with a cold engine after "specific pre-conditioning", the group said.

Friedrich is a co-founder of the Washington-based International Council on Clean Transportation (ICCT), which commissioned the original investigation that led eventually to the exposure of VW's test-rigging.