Swiss pharmaceuticals giant Roche announced on Wednesday it had launched a takeover bid for Illumina, after the US company specialising in life sciences rebuffed an offer.

Roche said it had made "multiple efforts" to start talks with Illumina to negotiate a deal but the US firm had not been willing to enter "substantive discussions."

"Roche has therefore decided to promptly commence a tender offer to purchase all of the outstanding shares of common stock of Illumina for $44.50 (34.2 euros) per share in cash," said a company statement.

This offer was "full and fair value" for the company, Roche chief executive Severin Schwan said.

He added: "It is our strong preference to enter into a negotiated transaction with Illumina, and we remain willing to engage in a constructive dialogue with Illumina to jointly develop an optimal strategy for maximizing the value of our combined business."

Roche said the offer represented a 64 percent premium over Illumina’s stock price on December 21, 2011 and put the value of the San Diego-based company at $5.7 billion.

A brief statement from Illumina acknowledged that Roche had made an "unsolicited offer" for the company.

It said the board of directors would review the proposal and make a recommendation in due course, adding that it had asked share holders to take no action in the meantime.

Illumina is a leading provider of integrated systems for DNA sequencing. Roche said the acquisition would strengthen its position in the life sciences and diagnostics market.

The company intends to combine its applied science business with Illumina and move the business area's headquarters to San Diego, California, while maintaining operations in Penzberg, Germany, where Roche Applied Science is currently based.

Illumina, which employs about 2,100 people, was established in 1998 and in 2010 posted a turnover of $902.7 million. It began trading on the Nasdaq in 2001.

In a letter sent to its CEO Jay Flatley, Roche group chairman Franz Humer said he was confident Illumina stockholders will find the offer "extremely attractive" and urged its board to negotiate the transaction.

He noted however a letter from the board on January 18 confirming Illumina's rebuffing of the deal.

Addressing media in a telephone conference call, Schwan said he believed sequencing would become an increasingly important technology and that Roche's expertise in the research and diagnostics market "would help accelerate the transition of sequencing into research and clinical diagnostics around the world."

"Combining the strengths of Roche and Illumina makes a lot of sense, both from a strategic and operational point of view," he said.

"We firmly believe that we will create value for both companies and this is why we approached Illumina in December last year."