LAS VEGAS » The top executive of the Volkswagen brand worldwide says he’s optimistic that U.S. environmental regulators will approve fixes within the coming weeks or months for diesel engines that cheat on emissions tests.

Brand CEO Herbert Diess said Tuesday night at the CES gadget show in Las Vegas that the company is having constructive discussions with the U.S. Environmental Protection Agency and the California Air Resources Board.

Diess says VW already has received approval to fix 8.5 million cheating cars in Europe. Repairs will start this month and most will be fixed this year.

But the U.S. cars are more problematic because they emit up to 40 times more toxic nitrogen oxide than allowed. About 500,000 cars are affected in the U.S., with a total of 11 million worldwide. Diess spoke as the company unveiled a concept of an electric-powered Microbus that could go into production in 2019.

U.S. fixes could be complicated and take several years. VW has admitted cheating on about 500,000 diesel cars nationwide by installing software that turns emissions controls on during government tests and turns them off on real roads.

Diess apologized for the scandal. “I’m optimistic that we will find a solution, we will bring a package together which satisfies our customers first and foremost and then also the regulators,” he said.

The U.S. Justice Department sued Volkswagen on Monday over emissions-cheating software, potentially exposing the company to billions of dollars in penalties for clean air violations.

The company is in the midst of negotiating a massive mandatory recall with U.S. regulators and potentially faces more than $18 billion in fines for violations of the federal Clean Air Act.

The company and its executives could also still face separate criminal charges, while a raft of private class-action lawsuits filed by angry VW owners are pending.

The company first acknowledged in September that the cheating software was included in its diesel cars and SUVs sold since the 2009 model year, as well as some recent diesel models sold by the VW-owned Audi and Porsche brands.