MUNICH -- General Motors Co. says a bid for Opel from Magna International faces hurdles while a rival offer from Belgian investor RHJ International would be easier to accept.

Writing on GM Europe's Web site, John Smith, GM's chief negotiator for the Opel sale, says Magna's bid "contained elements around intellectual property and our Russian operations that simply could not be implemented."

He adds: "GM has partners in other parts of the world who have joint ownership of these assets ... we simply could not execute the deal as submitted."

Smith said RHJ's bid would represent "a much simpler structure and would be easier to implement."

RHJ's offer would need less state aid and would keep GM's global alignments intact. "This remains a reasonable and viable option to be considered as the very difficult issues around the Magna negotiations continue to be worked," Smith writes.

Smith said GM does not have a favored bidder and is talking with Magna to formulate a bid that can be implemented.

"We remain fully open to working these issues to a conclusion, but we cannot say when that process will conclude, other than to say, 'as soon as possible,' " Smith says.

If it wins control of Opel, Magna wants to expand the brand in Russia, home of the supplier's bidding partner, Sberbank.

Russia's No. 2 carmaker, GAZ, is an industrial partner in Magna's consortium.

GM's Chevrolet marque is the top-selling foreign brand in Russia. The company would be reluctant to see Opel's technology used to strengthen a competitor.

Canadian auto-parts supplier Magna International reportedly has increased the amount of upfront capital it will inject into its offer for General Motors' Opel. No word on whether its competitor RHJ International has countered.

Unnamed government sources have told various news outlets that Magna is now offering 350 million euros ($700 million U.S.) of its own capital immediately and 150 million euros in a convertible bond.

Originally, Magna said it would invest just over 100 million euros in two payments of 50 million euro each along with another 400 million in convertible debt.

The German government, which has been asked for loan guarantees for any offer for Opel, wanted the bidders, now apparently narrowed to two, to boost their upfront cash investments.

Magna's offer includes participation by Russia Sberbank. Also in the running is Belgian-based private equity firm RHJ International. The German government reportedly prefers the Magna offer. So do Opel workers. Opel has asked employees to contribute nearly $2 billion in cost cuts between 2010 and 2014, the company's unions revealed Monday, so they want a say in the outcome.

GM supposedly favors the RHJ deal as a better business deal with fewer complications than Magna's bid.

GM's board will review the Opel bids on August 3. -- By Michelle Krebs