"Given the subdued growth prospects in regions outside China, there is no guarantee that 2015 will be a successful year, either for the industry or for VW," finance chief Hans Dieter Poetsch said on Friday.

Shares in Europe's largest carmaker plunged almost 6 percent before partially recovering to trade down 2.3 percent at 219.75 euros by 2.21 p.m., pulling the European autos index <.SXAP> down 0.4 percent.

Volkswagen said its 2015 group operating margin could be in a range of 5.5 to 6.5 percent, leaving its forecast unchanged.

But it raised its forecast for revenue, saying it could exceed last year's record 202 billion euros (146 billion pounds) by as much as 4 percent.

"I find the outlook very conservative. It's almost identical to last year's although currency markets are more positive than a year ago and demand for cars is also a touch better," said Metzler Bank automotive analyst Juergen Pieper.

The return on sales at the group, whose brands include Audi, Bentley and Porsche, rose to 6.3 percent last year from 5.9 percent in 2013, VW said.

Operating profit rose 8.8 percent to 12.7 billion euros, up from 11.67 billion in 2013 and slightly above analyst projections of 12.6 billion euros.

VW has said it is bracing for a tough year after sales at its namesake brand, accounting for about 60 percent of group deliveries, fell for a fourth month in January with demand shrinking in key European and Chinese markets.

(Reporting by Andreas Cremer and Edward Taylor; Editing by Maria Sheahan and Georgina Prodhan)