Siemens and the French government intervened in U.S.-based General Electric's plan to buy the power arm of France's Alstom on Sunday by offering an alternative tie-up between European "champions" and a pledge to act in France's national interest.

Though Alstom is privately owned, France's firebrand Economy Minister Arnaud Montebourg issued a stark reminder of the influence the government holds over a company that relies heavily on orders from state rail operator SNCF and partly state-owned utility EDF .

"GE and Alstom have their calendar, which is that of shareholders, but the French government has its own, which is that of economic sovereignty," Montebourg said in a statement.

On the day that GE boss Jeff Immelt had been due in Paris to thrash out a $13 billion deal for struggling Alstom's power turbines and grid equipment division, Montebourg provided the first official confirmation of the GE offer's existence.

He said it had been presented to him last week by Alstom Chief Executive Patrick Kron and related to "the purchase of the energy business of Alstom", which also makes railway rolling stock, as does Germany's Siemens.

By contrast, Montebourg said that an alternative Siemens proposal made public on Sunday was "about creating two European and global champions in the energy and transport domains - one around Siemens, the other around Alstom".

Alstom's board had been due to hold a meeting on Sunday to discuss the GE proposal, which sources said was worth $13 billion, before Siemens entered the fray with the announcement that it had written a letter to "signal its willingness to discuss future strategic opportunities" with the French group.

Siemens gave no further details on the letter and Alstom declined to comment. GE has remained tight-lipped on the subject since last week.

TRAINS PLUS CASH

However, a report on newspaper Le Figaro's website on Sunday said Siemens was offering Alstom half of its train-making business plus cash in exchange for its French rival's power turbines division.

Le Figaro said it had seen the letter and that it included a proposal for Alstom to take on Siemens' high-speed trains and locomotives arm, but not its metropolitan trains division. The paper gave no detail on the cash element of the deal.

A report in Germany's Handelsblatt outlined a similar scenario. The paper, which also said it had seen the letter, put the value of the proposed deal at 10-11 billion euros ($14-15 billion).

Though Alstom CEO Kron has said in the past he is not keen on creating a Franco-German train manufacturer, the French government has made it clear since the GE offer leaked last week that it wants to find alternatives to the U.S. group's proposal.

Montebourg said last week that he planned to meet GE boss Immelt and talks had been expected on Sunday. After news of the Siemens letter, however, an economics ministry spokeswoman said the meeting had been postponed for a few days.

The outspoken minister, promoted in a government reshuffle this month, has been a strong advocate for France's traditional caution about foreign takeovers in flagship industries.

Alstom makes TGV high-speed trains and is one of France's top private-sector employers. Though it is struggling with heavy debt and weak demand, political sensitivities run deep in France and a Siemens tie-up may be no more palatable to some than a deal with GE.

A decade ago Alstom was rescued by a state-backed restructuring when the country's president at the time, Nicolas Sarkozy, balked at the prospect of a Siemens acquisition.

POLITICAL OPPOSITION

"I ask you, Prime Minister, to please tell the shareholders and management of the groups concerned that this transfer of control is out of the question," former Socialist minister Jean-Pierre Chevenement said in an open letter to Manuel Valls on Saturday.

On Sunday, a statement from Marine Le Pen, leader of the right-wing National Front (FN) party that won widespread support in last month's local elections, said the government had "abandoned Alstom to be dismantled for American or German profit".

Alstom employs 18,000 people in France, about 20 percent of its total workforce, against GE's 10,000 French workers and Siemens' 7,000.

Sources have said a GE deal is backed by Alstom's main shareholder, French conglomerate Bouygues , which holds a 29 percent stake.

A deal to sell Alstom's power assets, which account for about 70 percent of total group revenue, would effectively break up the engineering group and leave Alstom as a pure transport business building its TGV trains, other rolling stock and transport industry equipment.

The power assets include turbines for coal, gas and nuclear plants, wind farms and systems for power transmission and distribution.

Before news of the potential GE deal, Alstom shares had slumped 20 percent in 12 months on concerns over its cash flow, prompting Bouygues to take a $1.9 billion writedown on its stake in February. ($1 = 0.7227 Euros)

(Additional reporting by Andrew Callus, Matthieu Protard, Yann le Guernigou and Natalie Huet; Editing by David Goodman)