French health-care group Sanofi has agreed to buy U.S. hemophilia specialist Bioverativ for $11.6 billion, its biggest deal for seven years and a major play to strengthen its presence in treatments for rare diseases.

Several analysts said the deal looked expensive, given uncertainties in the hemophilia market due to the arrival of new drug treatments, and Sanofi shares fell 4 percent on Monday, the worst performance on France's benchmark CAC-40 index.

The move comes at a time of renewed interest by large drugmakers in smaller biotech firms, with Celgene also announcing a $9 billion deal to buy Juno Therapeutics, and predictions by some experts that 2018 will see a substantial pick-up in mergers and acquisitions.

Sanofi has agreed to buy all of the outstanding shares of Bioverativ for $105 apiece in cash, a premium of 64 percent to Bioverativ's closing price on January 19.

Bioverativ, a maker of hemophilia drugs, was separated from Biogen early last year. Its shares surged to $104 in early U.S. trading.

The agreed transaction marks Sanofi's successful return to deal-making after its failure to land major takeovers in recent years. It is its biggest acquisition since the 2011 takeover of U.S. biotech company Genzyme for around $20 billion.

Sanofi lost out on buying California-based cancer specialist Medivation to Pfizer in 2016, and also missed acquiring Swiss biotech company Actelion, which was bought by Johnson & Johnson last year.

"With Bioverativ, we welcome a leader in the growing hemophilia market," Sanofi Chief Executive Olivier Brandicourt said.

The market dealing with treatments for hemophilia is an important one that is evolving rapidly as a new drug from Roche changes the landscape and gene therapy holds out the promise of a possible one-time cure.

Sanofi said the sector had around $10 billion in annual sales, with 181,000 people affected worldwide, and hemophilia represented the largest market for rare diseases, set to grow by more than 7 percent per year through to 2022.