Swiss drug maker Roche today increased its hostile cash offer for Illumina by 15 per cent, valuing the US-based gene sequencing company at about $6.7 billion.

''Based on our discussions with Illumina shareholders we have seen interest to accelerate the takeover process,'' said Severin Schwan, CEO of Basel, Switzerland-based Roche.

The sweetened offer came after Roche held ''a number of productive discussions'' with Illumina shareholders in recent weeks, said Roche.

Roche has now increased its offer from $44.50 a share or $5.7-billion to $51 a share, and Illumina investors have until 20 April to tender their shares to the revised offer.

The board of San Diego, California-based llumina had rejected Roche's initial offer made in January, forcing the Swiss pharmaceuticals giant to take its offer directly to shareholders.

"Roche's preference continues to be a negotiated transaction. We look forward to the possibility of a swift completion that offers immediate value to Illumina's shareholders," said Schwan.