* Would value SFR at around 15 bln euros - source
* Vivendi would keep 30 pct of merged company - source (Recasts with confirmation of talks, context, shares)
By Leila Abboud and Gwénaëlle Barzic
PARIS, Feb 24 (Reuters) - French media group Vivendi confirmed on Monday it had been approached by cable group Altice over a tie-up between mobile operator SFR and cable firm Numericable, but said it had not yet received any formal offer.
The proposal being discussed would value Vivendi unit SFR at around 15 billion euros ($20.6 billion) and would see Vivendi keep a stake of roughly 30 percent in the new company, a source close to SFR told Reuters earlier. A second source cautioned that it remained to be negotiated what proportion of the new company the two sides would own.
A deal would accelerate Vivendi's exit from capital-intensive telecoms as it tries to focus on pay-TV and music, and would enable SFR to spend less on rolling out high-speed fibre broadband in France by relying on Numericable's network.
Les Echos newspaper first reported on the talks on Sunday, saying the parties aimed to reach a firm deal in a few weeks. It would involve an issue of about 8 billion euros of debt, the paper added.
Analysts have estimated the potential financial benefits of the pairing could be worth as much as 6 billion euros.
A Vivendi spokesman declined to comment beyond saying that no memorandum of understanding had been signed for now. Both SFR and Altice, the holding company of founder Patrick Drahi, Numericable's largest shareholder, declined to comment.
A partial exit from SFR would cap a tumultuous period at Vivendi, once Europe's biggest media and telecom conglomerate built during an acquisition spree in the late 1990s.
The past two years have been a period of soul-searching for the company after it came to realise its various businesses did not make much sense together and Chairman Jean-Rene Fourtou lost enthusiasm for the telecoms business.
With the arrival in late 2012 of Vincent Bollore on Vivendi's board as the group's largest shareholder, the strategic review has gathered pace. It agreed to sell its second-biggest unit, Maroc Telecom, to Gulf operator Etisalat last year. The divestment is expected to close soon.
Drahi, Numericable's largest shareholder, has made no secret of his interest in a tie-up with SFR. The two sides held unsuccessful discussions in late 2012 but Vivendi was unconvinced by Drahi's proposal and price.
Since then, Drahi carried out initial public offerings of both Numericable and Altice to bolster his financial firepower and borrowing ability and to take another run at buying SFR.
Shares in Numericable were 4.3 percent higher at 30.74 euros at 1225 GMT. Vivendi shares were up 1.3 percent.
"There should be significant synergies from such an operation, namely with the migration of SFR fixed broadband customers to Numericable's cable network," Espirito Santo analysts wrote in a note.
A deal would allow SFR to use Numericable's lines into homes rather than renting those of its rival Orange. The tie-up would also lower the cost of rolling out high-speed broadband fibre, something Numericable has already heavily invested in.
Vivendi's former cash generating machine, SFR has been hammered by a price war started when rival Iliad undercut it with its low-cost "Free Mobile" offer, forcing it to spend money to try to keep clients.
Numericable, which listed on the stock market in November in France's biggest initial public offering since 2009, had previously been seen as a potential takeover target for SFR and rival Bouygues Telecom.
Altice, which also owns French and Belgian cable companies and mobile operations in Israel, listed on the stock exchange at the end of January.
Built via a decade of acquisitions, Altice is surfing a wave of investor interest in the European cable sector as a growing number of consumers turn to those companies for television and broadband at faster speeds and lower prices than from telecoms rivals. ($1 = 0.7275 euros) (Writing by James Regan and Natalie Huet; editing by Eric Walsh and Tom Pfeiffer)