The report, citing two people with knowledge of the talks, said Pfizer could present its bid to Ratiopharm's management as early as this week.
Several sources told Reuters in February that Teva Pharmaceutical Industries Ltd and Iceland's Actavis would be bidding for Ratiopharm. One source said at the time that the offer from Actavis was the highest, at close to 3 billion euros.
The deal appears set to become the biggest generics takeover since Teva's $7.5 billion purchase of U.S. rival Barr, announced in July 2008.
Pfizer declined to comment, while Ratiopharm was not immediately available for comment. (Reporting by Ritsuko Ando and Deena Beasley; Editing by Steve Orlofsky)