Regeneron Pharmaceuticals Inc said its longtime drug-development partner Sanofi SA aims to boost its stake in the U.S. biotechnology company through open market purchases of its stock.

The announcement on Monday lifted shares of Regeneron by 5.3 percent in morning trading.

Sanofi, which controls 16.7 percent of Regeneron according to Reuters data, said in an emailed statement that it has the right to increase its stake up to 30 percent as part of its partnership with the U.S. company. But it added that it had not announced any intention to take a controlling stake.

"We are very happy with the relationship with Regeneron, but we needed this technical filing to get freedom to operate," the statement said.

Sanofi's stake in Regeneron represents ownership of 15.82 million shares, according to Reuters data.

Regeneron said Sanofi is not allowed to buy more than 30 percent of the outstanding shares of its class A stock and common stock under "standstill" provisions of an agreement between the companies reached in December 2007. It was not immediately clear when the standstill provisions might lapse.

Sanofi intends that the value of its ownership of voting securities of Regeneron stock will surpass the $500 million U.S. antitrust notification threshold, Regeneron said in a press release.

Regeneron could not immediately be reached for further comment.

The drugmakers have a longstanding agreement to develop medicines together, including promising treatments for cholesterol and arthritis.

Regeneron is best known for its blockbuster drug Eylea for macular degeneration, which it co-markets with Bayer AG .

Morningstar analyst Lauren Migliore said it was unclear whether Sanofi was merely boosting its stake in Regeneron or eventually aimed for a controlling stake or outright ownership the company.

"I wouldn't want to speculate either way," Migliore said. "But Regeneron has a great drug portfolio and technology and a very strong partnership with Sanofi, so a merger would make sense."

But he also said a merger would be costly because acquirers typically have to pay a 30 to 100 percent premium for shares of attractive biotechs, and Regeneron's stock has risen more than fourfold in the past two years. The company's valuation has soared on booming sales of Eylea and greater awareness on Wall Street of its promising experimental drugs.

Shares of Regeneron rose $8.82 to $174.70 on the Nasdaq Monday morning. Sanofi shares were up 2.6 percent in Paris.

(Reporting By Ransdell Pierson and Debra Sherman; Additional reporting by Elena Berton in Paris and Zeba Siddiqui in Bangalore; Editing by Gerald E. McCormick and John Wallace)