Amazon said Thursday its 22-million song music catalog was now "optimized" for users of Apple devices, making it easier for iPhone owners to circumvent the iTunes store.

The move is part of a new initiative by the Internet retail giant challenging Apple's dominance of the digital music market.

"For the first time ever, iPhone and iPod touch users can discover and buy digital music from Amazon's 22-million song catalog using the Safari browser," Amazon said in a statement.

"Music purchases are automatically saved to customers' Cloud Player libraries and can be downloaded or played instantly from any iPhone, iPod touch, iPad, Kindle Fire, Android phone or tablet, Roku, Sonos home entertainment system, or any Web browser, giving customers the freedom to enjoy more music, from more devices than any other major cloud music service."

The move comes a week after Amazon launched a service that gives compact disc buyers instant copies of music in the Internet "cloud" in a challenge to iTunes.

Amazon AutoRip provides free MP3 versions of music on CDs bought from the online retail titan.

In the newest announcement, Amazon said its MP3 mobile website for iPhone and iPod touches is built on HTML5, which means customers can make purchases directly from the website.

Amazon offers some deals, including $5 albums, 69-cent songs, and free songs from artists on the rise.

According to research firm NPD, iTunes last year held a 64 percent share of the digital music market and 29 percent share of all music sold at retail. Amazon had 16 percent of the digital market, according to NPD's September survey.