German engineering giant Siemens is working on a joint bid with Mitsubishi Heavy Industries (MHI) for the energy assets of French group Alstom, the companies said .
However, sources within Alstom dismissed the joint offer, saying it would effectively mean a break-up of the French "national jewel".
Alstom , which favours a rival offer by US giant General Electric, "is sceptical and shocked... It would be a dismantling" of the group, the sources said.
Mitsubishi Heavy Industries (MHI) and Siemens said a statement 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."
The two groups would decide on the submission of a proposal to Alstom's board of directors "by ," the statement said.
The joint offer will rival a $17-billion counter-bid by US giant General Electric, which, while favoured by Alstom itself, 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 the country's biggest private sector employers with about 18,000 staff nationwide.
Alstom's energy unit, which builds generators, turbines and transmission systems and would complement GE's own power industry division, accounts for 70 percent of Alstom's business.
Once it has been sold, Alstom would be left with the railway equipment division that manufactures France's prized TGV high-speed trains.
While French economy minister Arnaud Montebourg favours a tie-up with Siemens, French Energy Minister Segolene Royal recent came out in favour of General Electric's takeover offer.
Siemens chief executive Joe Kaeser said it "appreciates MHI's commitment to join our efforts and I do look forward to working together with MHI to create a long-term oriented solution for Alstom, MHI and Siemens".
His counterpart at Mitsubishi Heavy Industries, Shunishi Miyanaga, said: "MHI has been invited by Siemens to join forces and we firmly believe that we can substantially contribute to a partnership solution for Alstom which will create value for all parties involved, including the country of France".