S&N owns several UK beer brands such as Kronenbourg 1664, John Smith's and Foster's, but the pivot of the takeover battle is its Russian and Eastern European joint venture Baltic Beverages Holdings (BBH).
The other 50% shareholder in BBH, which owns the Baltika beer brand, is Carlsberg.
Carlsberg and Heineken plan to divide up S&N's assets, with Carlsberg taking full control of BBH.
However, the Sunday Telegraph has reported that S&N has come up with a defence that would see it finance a bid for BBH by selling a 25% stake to a minority partner such as Anheuser-Busch, which would give the US giant a foothold in the fast-growing Russian market.
S&N has also talked to private equity funds Blackstone and Texas Pacific Group.
S&N is understood to value BBH at £4.6bn, while Carlsberg and Heineken's latest 780p offer gives BBH a value of £4.1bn.
Carlsberg and Heineken must come back with a new offer, expected to be 800p per share, by January 21 or walk away for six months under UK takeover panel rules.