BERLIN, March 8 (Reuters) - An expropriation of Rosneft's German assets remains an option for Berlin, a state secretary in Germany's Economy Ministry said on Friday, urging the Russian oil firm to swiftly propose potential buyers and not to return as owners down the road.

The comments by Michael Kellner come a day after Germany extended its trusteeship over Rosneft's local assets for the third time since 2022, stopping short of expropriation as Rosneft has signalled its willingness to find a new owner.

Kellner said Germany would not allow substitute owners for the assets - including a 54.17% stake in the Schwedt refinery in eastern Germany - that would allow Rosneft to buy back in at some point in the future.

"It is now up to the Russian side to present possible buyers. If these are investors outside the EU, we will carry out relevant checks in line with the Foreign Trade and Payments Act," Kellner said.

"The question of expropriation remains on the table. It is clear that we need a permanent solution for the three refinery sites," Kellner said, referring also to Rosneft's two other German holdings in the MiRo and Bayernoil refineries. (Reporting by Andreas Rinke; Writing by Christoph Steitz Editing by Ludwig Burger and Susan Fenton)