Novartis has been expanding rapidly

Novartis currently owns 42.2% of Chiron and is looking to seal control of the company and make up ground on larger rivals such as GlaxoSmithKline.

Novartis has offered $40 per share, 10% more than Wednesday's closing price.

Analysts said that while the move makes sense, the price may not be high enough to entice Chiron's other shareholders.

Chiron shares have fallen in recent months and investors may only be willing to part with their shares once they have rebounded and the offer price has been raised, analysts said.

'Better position'

Chiron has had difficulties producing flu vaccines after its plants in Germany and England suffered from contamination problems.

"We believe Novartis would be in a better position to help Chiron resolve its key issues as a wholly owned subsidiary," Novartis said.

Novartis has been spending heavily as it looks to move up the pecking order of world drugmakers - it is currently the sixth largest.

It is also looking for new drugs to boost its product line.

The company has already spent almost $8bn this year buying two generic drugmakers, and $660m acquiring the non-prescription drug business of rival Bristol-Myers Squibb.