The Obama administration pledged on Monday to stay out of General Motors' choice of a buyer for its European Opel unit, while union leaders in Germany put more pressure on the U.S. automaker to make a decision.

Negotiations over Opel, which General Motors Co is selling as part of a U.S. government-orchestrated restructuring, have dragged on for months, frustrating its 50,000 European employees.

On Monday, labor leaders for Opel's 25,000 workers in Germany protested the delay by rescinding an agreement to forego their vacation pay, demanding some 70 million euros ($100.1 million) in cash next week, sources familiar with the action told Reuters.

This is the first warning shot, said one of the sources, who requested anonymity.

GM had been expected to reach a decision on Friday on whether to sell Opel to a group led by Canadian automotive company Magna International, Berlin's favored bidder, or to Brussels-based investment firm RHJ International, but it failed to make a final decision.

Instead, the board asked the German government for more information about state financing for an RHJ bid, sources familiar with the meeting told Reuters on Friday.

GM emerged from bankruptcy protection in July with 60 percent government ownership, after receiving $50 billion in loans from U.S. taxpayers.

But White House Deputy Press Secretary Bill Burton said on Monday that President Barack Obama's administration had no plans to get involved in GM's decision regarding Opel.

The president's view is that decisions made on the day-to-day operations of General Motors should be made by the folks at General Motors. He never wanted to get into the auto business and he's happy for them to make their decisions and get back on their feet, Burton said at a press briefing in Oak Bluffs, Massachusetts, where Obama is vacationing.

NO STATE LOAN FOR RHJ

As German government officials grew increasingly anxious over Opel, Finance Minister Peer Steinbrueck called GM's delay annoying and urged a quick decision.

In an interview to be published in the Handelsblatt newspaper on Tuesday, Steinbrueck was quoted as saying the government had a clear preference for the Magna group and would not offer bridge financing to RHJ.

It's my impression that the management of GM likes the investor RHJ International because it would be easier for GM to buy Opel back in a few years, Steinbrueck was quoted as saying.

Opel's fate could influence Germany's federal election in September.

GM has said the RHJ offer would be easier to implement than the Magna plan, which involves an equal equity stake for Russian bank Sberbank and an industrial partnership with Russian automaker GAZ Group.

One concern analysts see for the Magna deal is GM's ability to control its intellectual property and vehicle technology in the Russian partnership.

German Foreign Minister Frank-Walter Steinmeier sought help from U.S. Secretary of State Hillary Clinton over the weekend to resolve the Opel issue.

Steinmeier was told she would only relay Germany's concerns to the U.S. Treasury Department, which oversees the GM investment through an autos task force. It was a real short conversation, said a U.S. State Department official, who asked not be named because he was discussing private diplomatic conversations.

AN EARLY TEST

The GM-Opel sale represents an early test of the U.S. government's relationship with an automaker in which it holds majority control. While no government employees sit on GM's board, the autos task force was active in vetting five new GM board members installed in July.

Any U.S. government pressure on GM regarding the Opel decision could cause broader concerns about federal influence in commercial decisions of a range of companies that have received taxpayer support.

It also could scare away investors the government hopes to court for a GM initial public stock offering as early as next year.

Erich Merkle, an analyst at Autconomy.com in Grand Rapids, Michigan, said Obama needed to resist political pressures surrounding the Opel decision.

If you are not interested in running an automaker, the government shouldn't be making this decision for General Motors, Merkel said.

'SPECTACULAR MEASURES'

In Germany, the union's action on Monday could be escalated if a decision on Opel does is not made soon. The person familiar with the matter said workers could ultimately demand a 4.2 percent wage hike from last year's collective bargaining agreement that they had agreed to waive for five years.

Opel labor leader Klaus Franz, speaking on German radio, threatened 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.

Ulrich Wilhelm, the spokesman for German Chancellor Angela Merkel, tried to calm the row at a government news conference on Monday, saying it was important that the union not resort to a confrontation with GM.

He added that the German government had invited a member of GM's board to Germany to discuss Opel this week, but played down prospects of a quick agreement.

(Reporting by David Lawder and Christiaan Hetzner; Additional reporting by Noah Barkin, Paul Carrel and Erik Kirschbaum in Berlin, Maria Sheahan and Angelika Gruber in Frankfurt, Kevin Krolicki in Detroit, Arshad Mohammed in Washington, and Patricia Zengerle in Massachusetts; Editing by Toni Reinhold

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