Sanofi-Aventis Chief Executive Chris Viehbacher has stepped up his campaign to win Genzyme by meeting shareholders of the U.S. biotech company face to face, the French company said Tuesday.

Genzyme last week rejected an all-cash $18.5 billion takeover offer from the French drugmaker, setting the stage for a potentially hostile takeover battle. But Viehbacher has made it clear he would rather get his opposite number, Henri Termeer, to the negotiating table and is keen to sound out investors before making his next move.

"He's in the U.S. and he decided to spend some time with shareholders to discuss the proposal," Sanofi spokesman Jean-Marc Podvin said by telephone from Paris.

ISI Group analyst Mark Schoenebaum, who attended a group lunch with senior management in New York on Tuesday, said Sanofi had told dozens of investors at the event it expected to have seen 50% of Genzyme shareholders by Wednesday evening and might then consider taking a next step.

The French group also signalled there was no absolutely precise way of valuing Genzyme and it wanted to know what willing Genzyme sellers would accept, adding it would not be "hard-headed," Schoenebaum wrote in a note.

But before taking any more steps, the company wanted to be sure it could realistically complete a transaction and wouldn't get stuck in "gridlock." The lunch meeting was addressed by both Viehbacher and Chief Financial Officer Jerome Contamine, Schoenebaum told Reuters.

Sanofi's current offer is worth $69 a share, but Genzyme stock is trading at around $71 and investors polled by Reuters on Aug. 30 said they would not accept anything under $75 per share.

Sanofi wants to buy Genzyme, a leading maker of drugs for rare diseases, to fuel growth as some of its key treatments lose patent protection. It would, however, be taking on a risk because of the manufacturing problems that have hit Genzyme's profits and share price in the past year.

As a result, due diligence for the deal - which could take as little as two weeks - is likely to focus on the likelihood of Genzyme winning timely regulatory  approval for its new Framingham manufacturing facility, Schoenebaum quoted Sanofi management as saying.

Sanofi's Podvin declined to comment on what was discussed with investors during Viehbacher's meetings in the U.S..