At least 10,000 Volkswagen more vehicles have been found to use devices to cheat emissions tests, according to US regulators.
The Environmental Protection Agency (EPA) said cars using Volkswagen's 3.0 litre diesel engines, built between 2014 and 2016 are affected. It includes Audi and Porsche models.
Volkswagen denied using the cheat device, and instead claimed its 3.0 V6 engines "had a software function which had not been adequately described in the application process".
The EPA said: "VW has once again failed its obligation to comply with the law that protects clean air for all Americans."
EPA assistant administrator Cynthia Giles said: "These tests have raised serious concerns about the presence of defeat devices on additional VW, Audi and Porsche vehicles. Today we are requiring VW Group to address these issues. This is a very serious public health matter."
Last week, Volkswagen posted its first quarterly loss in around 15 years as the immediate afermath of the full scandal, which involves more than 11 million vehicles, took affect.