Sanofi-Aventis declined to comment on Friday on a Bloomberg report that it was preparing an acquisition of $20 billion or more in the U.S. as shares in the French drugmaker fell. The group, headed by Chris Viehbacher, is actively looking for acquisitions to find new sources of growth as its own product pipeline is drying out and some research projects have been pulled.

"In big pharma, everybody is looking at everything and they all have big departments looking at possible acquisitions," said Raymond James analyst Thierry Verrecchia. Bloomberg reported on Thursday that Viehbacher briefed the board on the deal at a special meeting this week, citing two people familiar with the matter.

The process was at a "very early" stage and an agreement with the target, which was not identified, may yet not be reached, Bloomberg reported.

Earlier in the week, Sanofi bought U.S. biotech TargeGen, which is developing treatments against blood diseases, for as much as $560 million. Analysts cited Allergan and Biogen as possible targets, with a market value of $17 billion and $12.7 billion respectively.

Analysts cited Allergan, Biogen and Genzyme as possible targets, with market values of $17 billion, $12.7 billion and $13.5 billion, respectively.

A U.S.-based biotech acquisition would be a good strategic fit for Sanofi, said UBS analyst Fabian Wenner.

"What they need most is biological antibody competencies," Wenner said, adding that this put Biogen and Allergen in the frame, while Amgen or Bristol-Myers Squibb were too big for Sanofi.