Volkswagen managers are worried about traveling to the United States, a German newspaper reported on Saturday, saying US investigators have confiscated the passport of an employee who is there on a visit.

Volkswagen believes the investigators want to prevent the manager from evading questioning or criminal prosecution linked to the diesel emissions scandal, said the "Suddeutsche Zeitung" citing company sources.

"Volkswagen employees are still traveling to the United States. Everything else is speculation," said a spokesperson.

The company is under investigation in the United States and could face penalties of up to $18 billion after admitting it deliberately rigged emissions tests of diesel-powered vehicles.

Mary Nichols, head of the California Air Resources Board, which is investigating VW, has criticised the car-maker's handling of the scandal.

Citing a person with knowledge of the matter, the paper said it was now unlikely that new VW Chief Executive Matthias Mueller would travel to the United States in the second half of November, as planned.

"We need legal security here before he can fly to the United States," the paper quoted a person from the group management as saying.

There is no official plan for Mueller to travel to the United States, and VW has so far declined to comment when asked whether such a trip is likely.