Under takeover rules, AB InBev has until 14 October to make a firm offer.
The New York Post is reporting that SAB's management is "leaning towards fighting" the expected offer.
Its shares closed down 3.8% in London, but that is a small proportion of the amount they have risen since AB InBev's interest was revealed on 16 September.
SABMiller is still about 20% above its level before that announcement.
If a deal goes through, the merged company would produce one-third of the world's beer.
SABMiller makes Peroni and Grolsch, while AB InBev produces Budweiser and Stella Artois.
Also on Tuesday, as a result of the expected offer, the company brought forward its six-month trading statement, reporting a 2% rise in sales volumes for the three months to September.
Growth in Latin American and African drinks volumes offset declines in North America and the Asia Pacific region.
But reported revenues fell by 9% because of the depreciation of emerging market currencies against the US dollar.
Brewer SABMiller's shares have fallen after reports that it is likely to reject an offer from rival Anheuser-Busch InBev.