Apple Inc's iTunes media store emerged the second most popular music retailer in the United States behind Wal-Mart Stores.

The rankings were released by NPD Group a tracking firm based on its research on music that was sold last year by different media stores in the United States.

According to NPD, in 2007 music sales, legal music downloads account for 10 percent of music acquired in the United States although the increase never offset a decline in physical CD sales.

The iTunes music store has more than 50 million customers, with a catalogue of more than 6 million songs and has sold more than 4 million songs.

Apple said it plans to expand its footing in the video market.

We would like to thank the over 50 million music lovers who have helped the iTunes Store reach this incredible milestone, said Eddy Cue, Apple's vice president of iTunes.

We continue to add great new features like iTunes Movie Rentals to give our customers even more reason to love iTunes, added Cue.

Apple recently said it was expanding offerings from Hollywood studios after launching iTunes Movie Rentals, last month, with video from 20th Century Fox, The Walt Disney Studios, Warner Bros., Paramount, Universal Studios Home Entertainment, Sony Pictures Entertainment, Metro-Goldwyn-Mayer (MGM), Liosgate and New Line Cinema.

iTunes Movie Rental's goal is to sell more than 1,000 titles by the end of this month including about 10 percent in high definition video with 5.1 Dolby Digital surround sound.

iTunes users can rent the movies directly to widescreen television sets through Apple TV and also view the movies on iPod classics, iPod nanos with video and iPod touch.