The US government has said German car giant Volkswagen cheated on emissions tests for a second time.

The Environmental Protection Agency (EPA) said the firm installed software designed to defeat emissions tests on 2014 to 2016 vehicles with three-litre six-cylinder diesel engines.

The car maker previous acknowledged rigging emissions tests for four-cylinder diesel engines.

The agency said the software has a timer that makes the cars perform differently when being tested than they perform on the road.

The EPA said the cars give off up to nine times more nitrogen oxide pollution while being driven than they do when tested.

The violations cover about 10,000 vehicles, including the 2014 Touareg, 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8 and Q5.