The potential buyers include Belgium-based holding company RHJ International and Beijing Auto, a GM Europe spokesman said. Both companies have previously bid for Opel.
"GM is working through negotiations with Magna. It's typical in negotiations of this type that difficulties arise and we're committed to working through those difficulties to get a definitive agreement as soon as possible," the GM spokesman said.
"In the meantime, we continue negotiations with other potential partners including RHJ and Beijing Auto," he added.
The Financial Times reported on Tuesday that RHJ was close to a deal to buy a stake in Opel.
Magna chief executive Siegfried Wolf said his company "wants to come to an agreement by July 15."
"We want to make Opel into a company that stands along with its partners and distributors for success, growth and innovation," Wolf said.
Calls to Sberbank were unanswered.
Magna and Sberbank signed a nonbinding agreement last month that saw Sberbank taking a 35 percent stake and Magna a 20 percent stake in the General Motors subsidiary and invited Oleg Deripaska's GAZ to join the consortium in order to share its production facilities.
The deal was touted as a major coup for Russia's troubled auto industry, giving Opel the use of GAZ's Nizhny Novgorod factories while the Russian carmaker gained access to German engineering.
The deal also offered new hopes of improving Russia's auto-components industry, which remains a hurdle to greater localization of production.
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