The head of French pharmaceutical giant Sanofi-Aventis said Monday he was in no hurry to pursue a bid to acquire US biotechnology firm Genzyme.

Chief executive Chris Viehbacher, in an interview with the French financial newspaper Les Echos, said his intention at the moment was to confer with Genzyme counterpart Henri Termeer.

"We want to show our determination and our seriousness, without appearing to be threatening right away," he told the paper.

"There is still lots of time."

Sanofi-Aventis on Sunday unveiled an 18.5-billion-dollar (14.5-billion-euro) offer for Genzyme, hinting it might launch a hostile bid.

Sanofi said it was disclosing to Genzyme's shareholders the contents of its offer, which it initially made in July, after the company's management rejected it on August 11 and declined to enter into talks.

Sanofi said it preferred to enter into constructive talks with Genzyme's management but failing that it was "prepared to consider all alternatives to successfully complete this transaction."

A Sanofi spokesman stressed that for the moment this "is not a hostile offer ... but a proposal addressed the Genzyme's management."

Viehbacher said the Sanofi initiative did not amount to a takeover bid but was instead a "non-binding proposal."

He added that in pursuing Genzyme, Sanofi wanted to add "a growth platform at a time when we are losing major sales volume with the arrival in Europe of such generic medications as Plavix, Eloxatine and Lovenox from the United States."

Sanofi on Sunday said it wanted to make Genzyme its global centre for excellence in rare diseases and further increase its presence in the greater Boston area, where Genzyme is based.

Sanofi shares showed a gain of 0.14 percent to 45.32 euros on an overall Paris market that was up by 0.21 percent in early Monday trading.