UPDATE 2-German states unhappy with Fiat, unions to meet

* Hesse says Fiat has not done any due diligence * Kaiserslautern plant closure cannot be accepted by state

* Italian union not prepared to negotiate on job cuts

* German, Italian unions to meet next week

(Adds comments from German and Italian labour leaders)

WIESBADEN/MAINZ, Germany, May 8 (Reuters) - German regional governments anxious to keep thousands of Opel jobs warned Fiat (FIA.MI: Quote, Profile, Research) still had not presented a convincing strategy to unite the German carmaker under the leadership of its Italian rival.

Separately, Opel's labour leader said he had invited his counterparts from Italy's three major engineering unions to meet next week as anger seethed in the Mediterranean nation over constant speculation that domestic Fiat plants could be closed.

Hesse's conservative state premier, Roland Koch, questioned the accuracy of Fiat's plans since Chief Executive Sergio Marchionne had not been offered a peek inside the books of Opel, a subsidiary of GM GM.N that does not publish financial results.

"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 economics minister from the Liberal Democrats and Marchionne.

In an attempt to prevent Fiat from building too much momentum among Berlin's decision makers, Koch's state government 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.