The German government sought to calm a growing transatlantic row over the fate of carmaker Opel on Monday as unions threatened spectacular measures to force a decision by U.S. parent General Motors.

Negotiations over Opel, which GM is selling as part of a government-orchestrated restructuring, have dragged on for months, fuelling anger among its 50,000 European workers, half of which are in Germany.

GM was expected to reach a decision on Friday on whether to sell Opel to Canadian automotive firm Magna, Berlin's favoured bidder, or to rival RHJ International, but failed to make its choice clear.

The German government said on Monday it would invite a member of the GM board to Germany to discuss Opel this week, but played down prospects of a quick agreement.

A U.S. government official said President Barack Obama's administration was available to talk to the German government on the matter but made clear the decision on a buyer would be left up to GM.

German Chancellor Angela Merkel, who is under pressure to find a solution for the carmaker before a federal election next month, expressed regret about GM's delay over the weekend and said a decision was urgently needed.

Opel labor leader Klaus Franz then raised the stakes by threatening spectacular measures if GM did not make up its mind soon.

We have been calm so far, listened diligently and made comments, but that is over now, Franz said on German radio on Monday.

Merkel's spokesman Ulrich Wilhelm, speaking to reporters at a government news conference, tried to calm the row, saying it was important not to resort to confrontation.

I can understand that the Opel workers want to have clarity quickly about the way ahead, Wilhelm said. On the other hand, the decision is complex.

I think staying calm to preserve a good negotiating climate, is in the interests of all sides, he added.

Wilhelm said he had no indications that GM was rethinking its decision to sell Opel after recurring reports over the past weeks that some board members would prefer to keep the carmaker.

Were it to take that step, GM would likely need U.S. government funds. The carmaker has said publicly it has no plans to seek such funds.

PART OF BROADER OVERHAUL

Opel employs over 25,000 in four major plants in Germany that make models including the three-door Corsa subcompact and Zafira van. It has two factories in Britain that produce cars under the Vauxhall badge and sites in Belgium, Poland and Spain.

In the first half of the year, Opel and sister brand Vauxhall sold just over 560,000 cars in Europe, good for a market share of 7.6 percent, according to data compiled by the ACEA auto industry association.

The sale of Opel is part of a broader overhaul of GM, which emerged from bankruptcy protection on July 10.

Last week the humbled member of Detroit's Big Three automakers agreed to sell its Saab car business to Koenigsegg, a tiny Swedish luxury carmaker.

It is also in talks to sell its Hummer brand to China's Tengzhong though, like Opel, these have been plagued by delays.

Because Berlin is being asked to cough up billions of euros in finance guarantees to the eventual buyer of Opel, it has played a key role in the sale decision.

The federal government, German states with Opel plants and the German unions have made clear they favor the Magna offer, which is backed by Russia's Sberbank because they say it would allow the carmaker to grow and develop.

While Magna plans an aggressive push into the Russian car market, Belgium-based RHJ, led by turnaround specialist Leonhard Fischer, is offering a more traditional restructuring focused on lowering costs.

(Writing by Noah Barkin, Paul Carrel and Maria Sheahan; Editing by Richard Hubbard)

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