By Ian Youngs
Music reporter, BBC News
Spotify has gained more than a million UK users since it launched in October
Online music service Spotify is hoping to launch a mobile version and expand to the US by the end of the year.
The streaming service, which lets fans listen to more than three million tracks legally and free, has gained more than a million users in the UK.
Speaking about the plan to go mobile, Spotify founder Daniel Ek said: "We want to be everywhere. We won't only do one device."
But fans would have to pay to put Spotify in their pockets, he revealed.
The web-based service is currently free to users who accept adverts between their songs. A subscription option, which costs £9.99 a month in the UK, banishes the ads.
Speaking at The Great Escape music conference in Brighton, Mr Ek said the mobile offering would only be available to subscribers.
"Portability is an important aspect, [as is] interoperability with other devices," he said.
"That's definitely something we think is a premium product that people are willing to pay for - being able to bring the music with you or being able to have it working on your stereo."
The company is working on an iPhone application, but also wants to make the service available on other handsets, he said.
"I definitely hope that it will be before the end of this year that we'll do something which allows people to bring the music with them."
As well as the UK and Sweden, where Spotify was created, the current service is also available in Norway, Finland, France and Spain.
It has become a huge hit with music lovers in the UK since launching in October, but the recession and resulting advertising downturn have slowed its global expansion plans, Mr Ek said.
But he added: "My hope and ambition is that we'll see something at the end of the year, maybe the beginning of next year, in the US."
The 25-year-old entrepreneur launched the company in an attempt to provide a legal alternative to unlicensed file-sharing services, where artists, record labels and music publishers do not receive royalties.
He has not revealed how many users have paid to subscribe. But more extras will be added to tempt people to sign up for the monthly fee, he said.
Those will include better sound quality, the ability to listen to new releases before they hit shops, more social networking features, exclusive tracks and behind-the-scenes material from big-name artists.
In other money-spinning schemes, T-shirts, concert tickets and vinyl will be sold to fans as they are listening to their favourite bands.
The company is also expected to announce a deal with fellow streaming site Last.fm to provide song recommendations, making it easier for fans to find acts they like.
"We definitely want to have music recommendation," Mr Ek said. "I'd love to work with someone like Last.fm and I think in a couple of days you'll see some sort of announcement. Something will happen."
But video streams are unlikely to appear in the near future, Mr Ek said.
"It's something that we've considered but we're focusing right now on the audio side, and we're focusing on generating revenues.
"At the moment there are other features that are higher up in the priority list, but it's definitely not something that we've excluded."