NEW YORK/PHILADELPHIA: Biotechnology company Genzyme Corp is weighing an informal takeover approach from France's Sanofi-Aventis but is not looking to sell the company, a source familiar with the situation said on Monday.

Bloomberg reported earlier on Monday that Genzyme had rebuffed Sanofi's overture. Genzyme, however, has not responded to Sanofi or rejected the French company, a second source familiar with the situation told Reuters.

Sanofi and Genzyme declined to comment, but Genzyme shares rose another 7.8 percent on Monday and reached a level during the trading day not seen since February 2009.

Reports that Sanofi was making a run at Genzyme surfaced on Friday, sending the U.S. company's shares up more than 15 percent as investors figured the company would garner a hefty premium for its unique portfolio of expensive treatments for rare genetic disorders and pipeline of drugs in development.

Sources told Reuters that Sanofi had approached Genzyme two weeks earlier with an informal bid to buy the company and was awaiting a response.

Some company watchers say Genzyme shareholders may be willing to accept a price of $70 to $80 per share, particularly newer investors who were drawn to the company as it was targeted by activist investor Carl Icahn.

They might be willing to accept a narrower premium than long-standing investors, said Sam Isaly, managing partner at OrbiMed Advisors LLC.

"I suspect a figure in the $70 range will be enough," he told Reuters in an interview.