Magna International Inc. (MGA) and Italian Automaker Fiat SpA (F.MI) still need to improve their bids for General Motors Corp.'s (GM) Adam Opel GmbH unit, German Economic Minister Karl-Theodor zu Guttenberg said Wednesday.

Zu Guttenberg warned that an insolvency of the German carmaker remains an option as long as risks aren't shared and the U.S. doesn't back a trusteeship model for the company,

"At present, we have no such improvement that would allow us to say this (concept for the future of Opel) is more sustainable than previously," zu Guttenberg told reporters.

A top meeting Wednesday evening would be a success if the German government, state governments, the U.S. Treasury and GM agree on a trusteeship model for Opel and if investors would be willing to take on risks, he said.

-By Andrea Thomas, Dow Jones Newswires; +49 (0)30 2888 4126; andrea.thomas@dowjones.com