World number four mobile phone maker Sony Ericsson plans further to develop its PlayNow service, currently used for downloading ring tones, a spokesman for the Japanese-Swedish group said on Wednesday.

But he could not say what kind of further development that would entail, or when it would happen.

"We are developing the PlayNow service further, it is part of our bigger music stragegy," said spokesman Aldo Liguri. The company is half owned by Sony and Ericsson .

Many phone makers have invested in online music downloading services, and handset makers see digital music as one of the key drivers for selling more expensive new phones, as they try to hold up their average selling prices despite surging demand for cheap phones on emerging markets.

Nokia, the world's largest handset maker, said at the start of August that it was buying U.S.-based digital music distributor Loudeye Corp. for $60 million.

Sony Ericsson's current PlayNow facility allows users to hear a clip of a ringtone and then purchase if they want.