Swiss drugmaker Roche dangled the prospect of a higher offer before reluctant U.S. bid target Illumina on Thursday along with broadly flat first-quarter sales supported by its top-selling cancer drugs.

Genetics specialist Illumina has rejected Roche's $6.7 billion sweetened takeover offer and has also urged investors to vote against Roche's bid to get its own nominees on to the U.S. firm's board at a meeting next week to push for talks.

Chief Executive Severin Schwan told journalists that Roche would have to consider "all its options" if Illumina shareholders voted against its proposals at that meeting.

"If Illumina were to engage with us, we would consider any information supporting Illumina's contention that our offer undervalues the company and its prospects," Schwan said in a statement.

His comments echo those made in a letter to Illumina shareholders late on Wednesday in which Schwan also ridiculed Illumina's claim to be "the Apple of the genomics business".

Roche said its $51-per-share offer for Illumina - raised from an initial $44.50 - should be "more than enough" to begin negotiations with the maker of genetic analysis equipment that can help better identify which patients benefit from drugs.

Roche, the world's largest maker of cancer drugs, said first-quarter sales fell 1% to 11.03 billion Swiss francs ($12.05 billion), but rose 2% at constant exchange rates, meeting the average analyst forecast in a Reuters poll.

A return to growth for key cancer drug Avastin helped counterbalance a three percentage point hit to sales from the Swiss franc, while pharmaceutical sales fell 4 percent in western Europe due to pricing pressure from government cuts.

"Overall, a solid set of 1Q 2012 sales numbers, that have come in just ahead of consensus," said Jefferies analysts in a note. "The suggestion of further upside to the Illumina offer, should management start discussions, is not a surprise."

The bid for Illumina is part of Roche's move into more personalised medicines helped by advances in genetics research, and is spurred by the challenges facing the global drugs industry, including patent expiries and government price cuts.

Roche is better positioned than most to weather the storm, however, since its top-selling cancer medicines do not face imminent generic competition.

Even so, it has not been able to escape the impact of austerity measures in Europe, where many countries have imposed big cuts in their medicine bills. Roche said those pricing effects shaved 1.5% off European sales in the quarter.

Roche reiterated it hopes sales will grow in the low-to-mid single digit range at constant exchange rates this year, while core earnings grow in the high single-digits.

Sales of Avastin rose 1% to 1.385 billion francs, meeting average analyst forecasts and reversing several quarters of decline for what used to be Roche's top selling drug. Roche reiterated its peak sales forecast of 7 billion francs.

Avastin, hit last year when the United States revoked its conditional approval as a treatment for breast cancer, has been overtaken by Roche's other cancer drugs Rituxan and Herceptin, which both saw sales grow 7% in the quarter.

"Bull's eye numbers inline with expectations," said Vontobel analyst Andrew Weiss. "Oncology revenues (Avastin, Herceptin, Rituxan) overall performed about 1% better than expected."

Roche said Avastin's growth was mainly driven by increased use in lung and colon cancer, with growth in Japan and emerging markets offsetting a modest decline in western Europe, while sales stabilised in the United States.

Another highlight was a 32% jump in sales for hepatitis drug Pegasys, driven by increased demand in the United States for use in triple combination therapy.