General Motors Corp is trying to cement an agreement with Canadian auto parts group Magna to buy its European unit, Opel, and is willing to broaden discussions if necessary, GM's chief executive said on Friday.

We're just negotiating, we don't have an exclusivity agreement (with Magna), so we are open to discussion with others, Fritz Henderson said after a congressional hearing.

We are spending a lot of time with Magna trying to negotiate a more definitive agreement, Henderson said.

Earlier in Berlin, the premier of the eastern German state of Thuringia, Dieter Althaus, said he was optimistic Magna's takeover plans for Opel would go through.

I'm very optimistic that the negotiations will come to a good conclusion, Althaus told Reuters in an interview. I don't see any real risks at the moment either.

Althaus also said offers by other potential Opel investors provided no real alternative to the Magna deal.

(Reporting by John Crawley, Mari Saito and Rene Wagner)