Sanofi said it would consider all options to complete the transaction, hinting it would consider a hostile takeover bid.
The so-called bear hug letter aims to pressure Genzyme to respond to the offer or justify to its shareholders why it has not held negotiations.
It is our preference to work together with you and the Genzyme board to reach a mutually agreeable transaction, Sanofi Chief Executive Chris Viehbacher wrote to his counterpart at Genzyme, Henri Termeer.
Your continued refusal to enter into constructive discussions will serve only to further delay the ability of your shareholders to receive the substantial value represented by our all-cash offer, the letter said.
Genzyme was not immediately available to comment.
Sources previously told Reuters that Genzyme wants an offer of at least $75 per share before Sanofi could review its private financial records. Some shareholders want as much as $80 a share to clinch a deal.
We had one phone call in general terms and (Genzyme) immediately called a board meeting and said they weren't for sale without me even saying the price, Viehbacher said on a conference call.
When asked about how Sanofi will proceed, Viehbacher said: There is no reason to be discussing the next step when we are at this step now ... No reason to be discussing another price.
News of Sanofi's initial approach to Genzyme emerged late in July and the French drugmaker sent a written expression of interest on July 29.
Sanofi said that Genzyme rebuffed the offer on August 11, but after some persuasion, agreed to a meeting of financial advisers on August 24.
Sanofi said it had taken into account an anticipated recovery in Genzyme's performance in 2011, as the biotech company tries to rectify manufacturing problems that led to shortages of two of its top drugs -- Cerezyme, its drug to treat Gaucher disease, and Fabrazyme, its drug for Fabry disease -- and hit its stock price over the past year.
Sanofi said it had financing for the offer from BNP Paribas and JP Morgan and that it has not talked to Genzyme activist shareholders. Shareholders Relational Investors and Carl Icahn hold 3.8 percent and 4.9 percent of Genzyme, respectively.
Analysts have said they expect a deal to be finalized in the range of $74 to $77 a share.