Fiat has still not presented a convincing strategy to merge Opel with Fiat Auto, German regional governments have said.

Hesse state premier Roland Koch questioned the accuracy of Fiat's plans because CEO Sergio Marchionne had not been offered a look at Opel's books and the automaker does not publish financial results, Reuters reported.

"Fiat has so far not had a look into the data prepared for Opel Europe," the Hesse state chancellery said in a statement following a meeting in Frankfurt between Koch, his Liberal Democrats economics minister and Marchionne. Hesse is home to Opel's headquarters in Ruesselsheim near Frankfurt.

Attempting to prevent Fiat from building too much momentum amongst decision makers in Germany's federal government in Berlin, Koch's state government has said all bidders should have sufficient chance to present their concepts, which should be evaluated on the basis of possible threats to German jobs and the cost to German taxpayers, the report said.

Canadian parts supplier and auto assembler Magna International has been widely reported to be eyeing a minority stake in Opel, possibly cooperating with GAZ of Russia. GM also has said there are several investors interested in the brand which urgently needs finance to avoid running out of cash in the next few months.

In Mainz, the state government of Rhineland-Palatinate controlled by the labour-friendly Social Democrats attacked Marchionne's plans to close the nearby Kaiserslautern engine and parts plant that employs some 3,500 people, Reuters said.

"The question marks concerning the interests of Opel and its German production plants - above all Kaiserslautern - have grown rather than diminished," state premier Kurt Beck was quoted as saying in the regional capital, adding his fears were "very, very big."

In Milan, the leader of a major Italian union also , expressed concerns over Marchionne's plans.

"The union is not prepared to discuss a reduction of employment in Italy," FIOM secretary General Gianni Rinaldini told Reuters, confirming press reports that Fiat's plants in Pomigliano d'Arco near Naples and Termini Imerese in Sicily were most at risk.