Amazon’s MP3 music service, while still small, is growing. However, it’s not cannibalizing sales from Apple’s iTunes so far, according to a report from the NPD Group. The verdict is still out on whether Amazon can sustain the growth.

[The growth is a] "healthy indication that the digital music customer pool can expand into new consumer groups who have not yet joined the iTunes community," said Russ Crupnick, entertainment industry analyst with NPD.

Recently, NPD reported that iTunes had seized the number one position in digital music sales in the U.S., surpassing Wal-Mart. In February, Amazon MP3 took over the number two slot, according to NPD, when comparing individual music tracks.

In the report, cited by C|Net on Tuesday, Mr. Crupnick speculated that the Amazon growth may be due to curiosity, propelled by a Super Bowl commercial, and he isn’t sure of the long term prospects for Amazon. "We’ll have to wait to see what Amazon can do with those people," Mr. Crupnick said. "Does the traffic snowball or does it wane?"

In contrast to iTunes with only DRM-free tracks from EMI, Amazon’s MP3 service has unprotected songs from four major labels. Even so, Amazon is far behind Apple, according to NPD. They reported that iTunes’ sales remain more than 10 times Amazon’s.