The announcement by the German-Japanese giants however failed to gain traction with the French group, with a source close to Alstom saying it would effectively mean the company's break-up.
Siemens had been going head to head against GE for Alstom's energy assets.
But today, the German giant and Mitsubishi Heavy Industries announced that they have "joined forces in evaluating a potential proposal for certain assets of the French multinational conglomerate Alstom in order to strengthen the future position of Alstom, MHI and Siemens."
According to a source close Alstom, Siemens and Mitsubishi are planning to propose to form two entities following the takeover -- one comprising of Siemens and part of the Alstom assets and the other made up of Mitsubishi and the remainder of Alstom's energy assets.
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Another source closed to the French group however said Alstom "is sceptical and shocked" by the planned bid.
"It would be a dismantling" of the group, the source said, adding that it "is not a solution for (Alstom's) problems".
Alstom favours GE's bid, although it has said it would treat any offer by Siemens fairly.
However, GE's USD 17 billion bid has run into political opposition in France.
The French government views Alstom as a firm of national strategic importance and is concerned about safeguarding jobs as it battles record unemployment and declining industrial competitiveness.
Alstom is one of France's biggest private sector employers with about 18,000 staff nationwide.