Germany pressed the three bidders for General Motors unit Opel to improve their offers for the carmaker on Tuesday, saying they needed to assume greater risks and make credible commitments to preserve jobs and sites.

Economy Minister Karl-Theodor zu Guttenberg told reporters after meeting Fiat Chief Executive Sergio Marchionne in Berlin that the Italian carmaker's offer looked serious but that rival bidders Magna and RHJ International remained in contention.

There's no favorite, he said. Everyone knows that improvements are still necessary.

Fiat made an aggressive last-ditch push to convince the German government to back its bid for Opel ahead of a top-level meeting in Berlin on Wednesday where a preliminary decision on preferred bidders is expected.

Marchionne met with Chancellor Angela Merkel and Guttenberg on Tuesday morning to try to address German concerns about his ambitious plan to fold Opel into a transatlantic car empire that would also include U.S. carmaker Chrysler.

Fiat Vice Chairman John Elkann was also in the German capital to win support for the Italian carmaker's bid.

Merkel will host a meeting on Wednesday , which will include senior cabinet members, leaders from the states where Opel has plants, and top executives from GM and the three bidders.

As Fiat lobbied the government in Berlin, Magna and RHJ were preparing to present their plans to labor representatives at Opel's headquarters in Ruesselsheim, near Frankfurt.

The German government has made clear that GM will have the final word on who buys Opel, but Berlin will play a crucial role in that decision because it is being asked to cough up billions of euros worth of loan guarantees as part of any deal.

Opel has 25,000 workers at four plants in Germany and the carmaker's fate has become a politically charged topic of debate in Berlin ahead of a federal election on September 27.

Members of Merkel's Christian Democrats (CDU) are reluctant to see the state take on significant new commitments to save Opel and some are pressing her to allow an orderly insolvency of the carmaker rather than back a deal with one of the bidders.

Guttenberg said on Tuesday that insolvency remained an option if the bidders did not make credible commitments to preserve German jobs and showed a willingness to assume greater risks.

He said the government hoped to be able to settle on one or more preferred bidders on Wednesday, a step which could lead to further negotiations.

Pressure to choose a preferred bidder is building ahead of a June 1 restructuring deadline for GM set by the U.S. government, which could lead to a Chapter 11 bankruptcy filing by the Detroit-based carmaker.

Last week top ministers said the Magna bid, which is backed by Russian investors, looked to be the best of the three.

But since then, all three of the bidders have improved their offers, leaving a down-to-the-wire race for control of Opel, a group that traces its roots in Germany back to the 19th century.

(Writing by Noah Barkin; Editing by Jon Loades-Carter and David Cowell)