Berlin (AFP) - EU officials tipped off a US environmental group about Volkswagen's emissions cheating scam, an American official told German media, as the embattled company said the widening scandal had hurt sales.

"Officials from the EU informed the US environment group ICCT about the possible cheating on nitrogen oxide emissions," Mary Nichols, who heads the California Air Resources Board, told WirtschaftsWoche.

The weekly reported that the European Commission had been told by a Volkswagen supplier of the cheating as early as in 2011.

But the commission failed to act, prompting frustrated officials to pass the information on to US-based environmental group The International Council on Clean Transportation which in turned informed US authorities.

"If this information is confirmed, that means that the former and current industry commissioners, respectively (Antonio) Tajani and (Elzbieta) Bienkowska, had hidden the facts and lied before the European Parliament," said two Greens European lawmakers Yannick Jadot and Karima Delli in a joint statement.

Volkswagen is struggling to cope with the biggest crisis of its history over its admission in September that it had fitted 11 million vehicles with devices designed to cheat pollution tests.

The so-called defeat devices turn on pollution controls when the car is undergoing testing, and off when it is back on the road, allowing it to spew out harmful levels of nitrogen oxide.

The revelations have sparked investigations in several countries but they turned out to be only the beginning as Volkswagen last week admitted that it also understated carbon emissions, for 800,000 vehicles.

On Friday, the company said the revelations have had an impact on car sales, with demand for the group's 12 brands -- including Audi, Skoda and Porsche -- down 3.5 percent to 831,300.

And for cars bearing the VW badge, sales fell 5.3 percent for the month.

"The Volkswagen passenger cars brand is experiencing challenging times. We not only face the diesel and CO2 issues but also tense situations on world markets,” said Juergen Stackmann, chief of the Volkswagen brand.

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"In western Europe, the temporary sales stops for vehicles affected by the diesel issue had an impact on sales," he added.

Deliveries of VW-brand cars slipped 1.3 percent in western Europe compared to a year ago, with weakening demand seen in Germany, Spain and Italy.

And for the group, sales for western Europe were down 1.2 percent.

VW group's sales were however lifted by its biggest market China, where deliveries rose 1.6 percent to 312,200 cars for the month.

The emissions cheating revelations have come at a heavy cost for the group, which booked its first quarterly loss in more than 15 years as it set aside 6.7 billion euros to cover the initial costs of the scandal.

Its market capitalisation has also been slashed by nearly 40 percent since September, when the scandal first broke.

On Friday, the group's shares were trading up 1.47 percent at 96.79 euros.