BERLIN, March 27 (Reuters) - Volkswagen's software partnership with Rivian, a cornerstone of CEO Oliver Blume's turnaround strategy for the German auto group, has passed a key milestone, bringing the U.S. partner closer to its next tranche of funding.

Winter tests of the first vehicles using the software have been successfully completed, Volkswagen said on Friday.

"We're accelerating towards the future," Blume said.

The joint venture aims to develop a software platform to underpin a model revamp across Volkswagen's core brand as well as its U.S. pickup truck business Scout and premium subsidiary Audi.

By joining forces with Rivian, a California-based maker of electric vehicles, Blume sought outside help following years of problems at Volkswagen's own software arm Cariad.

The German carmaker is under pressure to catch up in a race with data and software-savvy players like Tesla and China's BYD.

In November 2024, Volkswagen agreed to pay $5.8 billion under the collaboration by 2027.

This included a $1-billion tranche expected this year upon completion of "technological milestones".

A spokesperson said at this time the company would not comment on the details of the transaction.