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.

Mr Keller, who joined Motorola in October 2006 to run global marketing and public relations, will be replaced by Jeremy Dale, who will manage handset marketing, and Eduardo Conrado, who will focus on business customers. Both are marketing professionals.

While Motorola paid tribute to Mr Casey’s contribution since his recruitment from HJ Heinz, it added: “Marketing is vital to Motorola, and this restructuring will ensure that our marketing efforts are even more closely aligned with our businesses.”

Motorola has struggled to restructure its lossmaking mobile business and bring new products to market after its strategy, pegged to the success of the Razr phone, fell apart 18 months ago. Ed Zander, former chief executive, resigned last year.

While its main rivals boosted sales in the fourth quarter, Motorola’s phone shipments plunged 38 per cent and its share of the global phone market has fallen from over 21 per cent to about 13 per cent.

Mr Brown has reshuffled his senior management team and named a new chief financial officer.

Last month, he took direct control of the mobile phone unit and told analysts this week he is seeking a full-time leader for the mobile phone business with consumer electronics knowledge. “We have got to get that stabilised,” he said.

Mr Brown, who faces a proxy battle with Carl Icahn, the Wall Street investor, at the group’s forthcoming annual meeting, is in the midst of a strategic review of the core mobile phone business, including its possible sale or spin-off.