Motorola, the US mobile phone maker, said on Thursday that Casey Keller, its chief marketing officer, had resigned less than 18 months after taking the job.

Mr Keller is the latest of a string of senior executives to leave the Schaumburg, Illinois-based group in the past year. His departure underlines the turmoil at Motorola as Greg Brown, recently appointed chief executive, battles to return the mobile unit to profit and halt the slide in market share.