French pharmaceutical group Sanofi-Aventis on Monday extended its unsolicited $18.5 billion takeover offer for the biotech company Genzyme Corp., but the U.S. company called it insufficient and recommended that its shareholders reject it.

Sanofi's previous tender offer for Genzyme shares expired just before midnight Friday. The French company has extended the $69-per-share offer until the end of the day Jan. 21 in New York.

Sanofi, the world's fourth biggest drugmaker, said in a statement before the open of the Paris stock exchange that 2.21 million Genzyme shares were tendered by Friday — or about 0.9 percent of the U.S. company's outstanding shares.

Sanofi spokesman Jean-Marc Podvin said such a low percentage of acceptance was "typical" in such tender offers, adding that executives at the French company "are not surprised."

"The key message is that we want to engage in a constructive dialogue with Genzyme to reach a transaction," said Podvin, insisting on the "strategic fit" that it would offer.

But within hours, Cambridge, Mass.-based Genzyme, which specializes in drugs for rare diseases, rejected the offer. The company has indicated it would fight Sanofi's hostile takeover effort and last month estimated its own shares are worth $89 a share.

Genzyme shares rose 45 cents to $70.27 — above the offered price — in afternoon trading in New York while U.S. shares of Sanofi rose 23 cents to $32.94.