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.

"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 considerable number of questions remain open, with all investors. And Magna as well as Fiat have to make concessions in some key areas."

A meeting late Wednesday 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.

"The main aim of the negotiations will be to limit risks which the German taxpayer might have to take on as a result of bridge financing or further guarantees," he said. "For this, a considerable degree of accommodating is necessary, mainly from the potential investors and the American government and General Motors."

He said it can't be expected the German government will express its preference for one single bidder later Wednesday but it might be best to keep several investors in the game.

The U.S. Treasury and GM still have to answer several key questions, he said.

"We urgently need this agreement with the U.S. side," he said. "If this agreement can't be reached, the security of the model isn't guaranteed. And if the security if the model isn't guaranteed that we have no security for taxpayers. And in such a case, all option are open, including an orderly insolvency."

Magna has teamed up with Russian automaker OAO GAZ Group (GAZA.RS) and state-controled OAO Sberbank (SBER.RS) in its bid. Financial investor RHJ International S.A. (RHJI.BT) has launched a rival offer, while Chinese automaker Beijing Automotive Industry Corp. (BAIC) emerged as a possible fourth bidder Tuesday.

Zu Guttenberg called the Chinese bid, which was submitted on two pages, "upgradable" and said this would require time.

Commenting on the decision of Opel's supervisory board to officially recognize the transfer of European plants, rights and patents from GM to Opel, which makes a separation of Opel from GM more likely, zu Guttenberg said this still requires an approval by GM and the U.S. Treasury and that is why it would be premature to laude the decision taken by the supervisory board.

Web site: www.bmwi.de

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