BMW and Audi have expressed doubts that the proposed DTM America series can go ahead without the involvement of a US manufacturer.
BMW Motorsport boss Jens Marquardt and Audi Sport head of DTM Dieter Gass believe that the participation of a domestic manufacturer is vital if the series proposed for 2017 is to happen.
Marquardt said that persuading a US manufacturer to get involved was the most significant hurdle to be cleared on the road to a North American series running for cars conforming to the rules of the DTM and Super GT in Japan.
"The main task is to see if we can get an American manufacturer to buy into the idea," he told AUTOSPORT.
"To come and race in America for one or two races is one thing, but if you want a series that runs to 10 or so races and you want it to be appreciated by the fans, you need a US manufacturer to be involved."
Gass added: "We, at very least need the Japanese manufacturers, but it would be better to have a domestic manufacturer.
"A series with only German manufacturers would be very difficult to imagine."
None of the US 'Big Three' marques - General Motors, Ford or Chrysler - has come out in support of DTM America.
GM Racing boss Mark Kent questioned whether his company needed an additional programme at a time when it is racing in two fronts in the new United SportsCar Championship.
"We keep watching it to see how it develops, but right now we are quite happy with our Corvette Racing programme and our Corvette DPs," he said.
"We have a great marketing platform in the USC, so there would have to be compelling reasons to add some kind of DTM programme to that."
DTM boss Hans Werner Aufrecht believes that the moves towards the creation of an international rulebook for DTM-style cars would offer a persuasive case for a US car maker.
"There's no doubt that the concept behind the technical regulations and the option to use a car built according to these regulations in Europe, Japan and the USA against other globally-renowned car manufacturers will be attractive for American companies," he said.
"The product must convince the manufacturers that getting involved pays off, and I'm confident that this is going to happen."
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