MUNICH -- General Motors today said its German subsidiary Opel is not for sale.

The company's comments came after German solar company SolarWorld announced

earlier today a surprise plan to offer to buy Opel's four German plants as well as its research center in Rüsselsheim, Germany.

"This is pure speculation. Opel is not for sale," GM Europe spokesman Nelson Silveira told Automotive News Europe. He declined to comment further.

SolarWorld CEO Frank Asbeck told Reuters his company's offer for the Opel plants was serious, but declined to detail his strategy for Opel following a potential takeover.

The Bonn-based company said in a statement it wanted to turn Opel into the first "green" auto company in Europe, making its offer conditional on a complete separation of Opel from its ailing U.S. parent General Motors.

SolarWorld said it would also seek compensation payments for Opel's German staff positions totaling 1 billion euros -- matching the figure the car maker had said it would seek in state funding.

"SolarWorld can provide Opel 250 million euros in cash and 750 million in bank credit lines subject to a guarantee from the federal government," the company said.

Its CEO Frank Asbeck told Reuters: "We've made a serious offer to General Motors to acquire the four German sites as well as the research and development facility in Ruesselsheim."

"We're waiting for a response from GM. Big companies work slowly." Asbeck added: "This is no gag. It's a serious offer."

Analysts were also dismissive of the SolarWorld plan, which comes days after Opel announced it was seeking about 1 billion euros in aid from the German government because its financial situation is threatened by the GM's U.S troubles.

"Neither the government nor SolarWorld can separate Opel from GM. I think SolarWorld's plans are unrealistic," said Daniel Schwarz, an analyst at Commerzbank.

Christian Breitsprecher, an analyst at Sal Oppenheim, called the plan "pretty crazy."

SolarWorld is one of the three largest solar energy groups in the world and has more than 2,000 employees. It had sales of just under 700 million euros last year.

It was founded in 1998 in Bonn by Asbeck. The firm benefited enormously from new energy laws set up by the Social Democrat-Greens government after it took power in 1998.

The company's announcement came as Opel vowed to slash production to help it cope with a deepening crisis in the auto industry that is sweeping manufacturers across the globe.

The head of Opel, Hans Demant, told the Wednesday edition of Germany's Frankfurter Allgemeine Zeitung that the company was preparing steps to help it weather a demand slump in the wake of the financial crisis.

"We are preparing ourselves for bad times and are lowering our volume plans for 2009 by a good 10 percent," he said.

Opel is the first European carmaker to seek government help. It has blamed its troubles on those of GM, which itself is lobbying the U.S. Congress for government bridge loans to survive the downturn.

German Chancellor Angela Merkel met with Opel representatives on Monday. She has said the government is ready to help Opel, which employs about 25,000 in its German plants in Ruesselsheim, Bochum, Kaiserslautern and Eisenach.

But she has described Opel as a special case and played down the possibility of broader government help for the auto sector, despite calls for aid to be extended to other companies.

Germany's state and federal governments have said they will decide by Christmas whether to extend a guarantee of 1 billion euros to Opel, should GM file for Chapter 11 bankruptcy protection in the U.S.

Opel has been part of GM since 1929.

Reuters contributed to this report

COPING WITH CRISIS

The company's announcement came as Opel vowed to slash production to help it cope with a deepening crisis in the auto industry that is sweeping manufacturers across the globe.

The head of Opel, Hans Demant, told the Wednesday edition of Germany's Frankfurter Allgemeine Zeitung the company was preparing steps to help it weather a demand slump in the wake of the financial crisis.

"We are preparing ourselves for bad times and are lowering our volume plans for 2009 by a good 10 percent," he said.

Opel is the first European carmaker to seek government help. It has blamed its troubles on those of GM, which itself is lobbying Congress for government bridge loans to survive the downturn.

German Chancellor Angela Merkel met with Opel representatives on Monday. She has said the government is ready to help Opel, which employs about 25,000 in its German plants in Ruesselsheim, Bochum, Kaiserslautern and Eisenach.

But she has described Opel as a special case and played down the possibility of broader government help for the auto sector, despite calls for aid to be extended to other companies.

"I don't share the view that Opel is a special case," Peter Mueller, the conservative premier of the state of Saarland, told German public television station ZDF.

"There should be help for all," Mueller said, adding that aid should not be limited to the auto industry.