The German carmaker, which triggered the biggest scandal in its history with last year's admission that it rigged U.S. diesel emissions tests, began a European recall of models in late January. It said in February that software updates on the affected 1.2-litre, 1.6-litre and 2.0-litre vehicles, as well as hardware fixes for about a third of the 8.5 million cars, would be completed by the end of the year.

But Germany's KBA motoring regulator has since March held up a recall of VW Passats. A source at VW said the KBA was concerned that the proposed fix to make the cars comply with emissions regulations would lead to an increase in fuel consumption for the Passat.

VW sales chief Fred Kappler said on Tuesday the carmaker is counting on the KBA approving technical solutions for the Passat in coming weeks. He said that the majority of the 8.5 million cars could be repaired in 2016 but an unknown number of vehicles would not be fixed until next year.

"We are in deep discussions with the KBA," Kappler said on an earnings call, noting that the German transport regulator had so far approved fixes for the VW Golf and Amarok and the Audi A4 and A5, among other models.

"We are quite optimistic about getting approval (for other models) in coming weeks," Kappler said, without elaborating.

VW remains mired in its emissions test-rigging scandal four months after its European recall got underway. It had hoped that a swift completion of vehicle repairs in its home region could serve as a precursor to recovering from the crisis.

(Reporting by Andreas Cremer; Editing by Tina Bellon and Susan Fenton)