Volkswagen says 2016 diesel models have suspect software that could help exhaust systems run cleaner during government tests.

Europe's largest car manufacturer said the issue involved the vehicles' "auxiliary emissions control device".

It operates differently from the so-called "defeat" device that last month sparked the cheating scandal engulfing the carmaker.

VW said the latest matter was revealed last week to the US Environmental Protection Agency and California regulators.

The software makes a pollution-curbing catalyst heat up faster to enhance performance of the device that breaks down smog-causing nitrogen oxide in the vehicles.

VW spokeswoman Jeannine Ginivan said: "This has the function of a warm-up strategy which is subject to approval by the agencies.

"The agencies are currently evaluating this and Volkswagen is submitting additional information."

The German carmaker last month admitted its 2009 to 2015 models were using a "defeat" device to bypass emissions testing.

The controversy has battered VW's share price, forced out its chief executive and prompted investigations around the world.