Despite only being live for a fraction of the time, Amazon's MP3 music download store is rapidly catching up with Apple's iTunes.

New data shows that Amazon is now second behind iTunes in the American download market.

Launched in the States in September, the web-based offering is the highest-ranked store to offer all its tracks without digital rights management.

While Amazon offers 4.5 million songs compared to Apple's 6 million (only 2 million of which are DRM-free), Amazon's catalogue includes rarer online finds such as Radiohead's back catalogue.

Amazon is also the only store to be able to offer DRM-free music from all four major labels - higher quality tunes that can be played on any player.

That - and competitive pricing - Amazon undercut Apple by as much as 40 cents per song at launch - are thought to be the reasons for the new service's success.

An international roll-out of the Amazon MP3 store is planned, but no timescales have yet been confirmed.